The Chemistry Help Blog

Apologies to any askers! My computer is broken for the foreseeable future, but I hope to be back...

Fri, 15 Mar 2013 07:48:43 -0400

Apologies to any askers! My computer is broken for the foreseeable future, but I hope to be back around exam time!

Much love,

Chemistry Help

Iron pyrite, often called "fool's gold" has the formula FeS2. If you could convert 15.8 kg of iron pyrite to iron metal, what mass of the metal would you obtain?

Wed, 27 Feb 2013 21:53:27 -0500

Iron pyrite has a molar mass of 119.97 g/mol.

The mass of a mol of iron is 55.85 g/mol.

55.85 / 119.97 = 46.55% iron by mass

15.8 kg * .4655 (part of a whole that is iron) = 7.35 kg iron metal.

Explain how carbon tetrachloride is nonpolar even though each C-CL bond is a polar covalent bond?

Mon, 25 Feb 2013 22:46:58 -0500

Carbon tetrachloride is nonpolar because all of the Cl atoms and their electrons push themselves an equal distance away from each other - so even though their individual bonds are polar covalent, the carbon tetrachloride molecule overall is balanced.

Look here:

and here:

That tetrahedral structure is nonpolar.

Many fast foods such as French fries are cooked in oils rather than in water. Why is this done and how could frequent consumption of these foods be a health concern?

Sun, 17 Feb 2013 12:45:42 -0500

1) We, as humans, like oils. They’re fats, and fats are full of calories, which we need to stay alive. Frying in oil also represents a faster and cheaper method than the time it takes to cook in water.

2) We do love fats and oils because of their calorie content, but fried foods represent a wealth of calories and a dearth of nutrition content. They may have simple carbohydrates, which are great if immediately converted into energy (as energy was processed for much of human development) but little to none of the other vitamins, minerals, proteins or fibers it takes to really keep us going.

Also, fried foods are usually heavily salted, and very few middle-and-upper-class first-world civilians need more sodium. Unfortunately, since food cooked in oil and salt is cheap, it’s predominately eaten most by those who can’t afford anything else.

Guys! Guys, they’re making a Chemistry series on Crash...

Tue, 05 Feb 2013 01:18:51 -0500

Guys! Guys, they’re making a Chemistry series on Crash Course! Guess who’s stupidly excited?

Tune in, if you haven’t. Hank Green is pretty great.

icerosephoenix asked: Calculate the value of a silver dollar using the following information: (a) 20...

Wed, 23 Jan 2013 13:25:36 -0500

icerosephoenix asked:

Calculate the value of a silver dollar using the following information: (a) 20 silver dollars weight one pound, (b) each silver dollar is 90.0% silver and (c) pure silver is selling for $29.53/oz

Well,

There are 16 ounces in a pound, so $29.53 * 16 ounces/pound = $472.48/lb
1 silver dollar * (1 lb / 20 silver dollars) = .05 lbs
.05 * .9 parts silver/silver dollar = .045 lbs of silver
.045 * $472.48/lb = $21.26 / 1 silver dollar

One liter of an ideal gas at 0 degC and 10 atm was allowed to expand to 1.89L against a constant external pressure of 1 atm at a constant temperature. The enthalpy change is -901 J for this process. Calculate q, w, and delta E.

Fri, 18 Jan 2013 01:46:16 -0500

Well, first off, we can figure out delta E (in kJ/mol) using PV = nRT

(10)(1)=(x)(.0821)(273)

x = .446 moles of gas

-901 J = -.901 kJ / .446 moles = -2.021 kJ/mol

q = heat supplied = amount x specific heat x temperature change

and

w = work done to system = - (work done by system) = - P DeltaV

In this case,

-10 atm * .89 L = -8.9 L*atm * 101.3 (the unit of conversion from L*atm to J) = -901 / 1000 = -.901 kJ

-.901 / .446 = -2.021 kJ/mol!

Isn’t that nifty? See, q is zero because there is no temperature change. I mean, you can figure out

DeltaE = q + w

but here it’s really unnecessary. Sorry for lateness, as it’s finals week around here.

What property of metals coincides with a low value for specific heat? Explain how the specific heats for metals relate to metallic bonding.

Thu, 17 Jan 2013 08:07:58 -0500

A low specific heat means that the metal is more conductive, or capable of passing on electrons. In metallic bonding, metal ions are present in a sort of sea of localized electrons, and since metallic substances are usually dense, it’s easy for the molecules of metal to vibrate and excite its neighbors.

44g of carbon dioxide occupies 22.4L at STP. a what is its density at STP. b what is its density at 300K and 123.4kPa. c At standard temp, what pressure is required for a density of 1.0g/L Thank you

Mon, 14 Jan 2013 21:38:16 -0500

a) What is its density at STP?

Density is g/L, so 44g / 22.4 L = 1.964 g/L

b) What is its density at 300K and 123.4 kPa?

For this, let’s use the equation PV = nRT, or

(pressure in atm)(volume in L) = (moles)(constant, .00821)(temperature in K)

123.4 kPa = 1.218 atm

We also know there is 1 mole of CO2. You can figure this out 1) by dividing 44 g by the molar mass of CO2, or 44 grams, or 2) by simply knowing 1 mol of any gas at STP occupies 22.4 L.

So, using PV = nRT,

(1.218)*(V) = 1*.00821*300

V = 2.022 L

To get the density, again divide grams by liters, or 44g / 2.022 L

Density = 21.761 g/L

c) At standard temperature, what pressure is required for a density of 1.0 g/L?

Alright, we have 44 grams.

44/x = 1

x = 44 L.

Use PV = nRT again, with P as the variable.

P * 44 L = 1 * .00821 moles * 273 K

P = .0509 atm = 5.157 kPa.

For those wondering, 1 atm = 101.3 kPa.

I really don't understand how to name covalent compounds, acids, and hydrates! If you could help me out that'd be awesome. I'm in Chemistry H and have a test on it on Monday!

Wed, 09 Jan 2013 21:04:39 -0500

Well, I know exam season is coming up for most high schoolers, and these are the sort of topics that are hard to explain in text, so I’ll link you to a few good videos:

naming covalent compounds

naming acids

more naming acids

naming hydrates

Hopefully these will help you out!

My learning tip:

Make a chart of the naming rules for covalent compounds, acids and hydrates, and complete a few worksheets until you’ve committed the rules to memory.

A good trick to remember for naming acids:

If you’re having trouble with suffixes on the ends of polyatomic acids, just remember: ‘I ate it, and it was icky’.

I.e, a chlorate acid would be chloric acid.

I'm taking AP Chem right now and it's literally killing me. Do you think you could explain the process of setting up a net ionic equation?

Tue, 08 Jan 2013 21:52:17 -0500

Well, first, I hope you’re all right. Second, sure! Net ionic equations are easy. ‘Net ionic’ means you only keep the parts of the equation that change. It’s essentially the process of throwing out spectator ions, or ions that are unchanged by the reaction.

ie:

Let’s react a sodium chloride solution (NaCl aq) with silver nitrate (AgNO3 aq).

The first equation you might write out would be:

NaCl (aq) + AgNO3 (aq) => NaNO3 (aq) + AgCl (s)

By looking up solubilities, I can see that NaCl, AgNO3 and NaNO3 are all soluble in water, meaning they break up into their ions. So the ionic equation of the above is:

Na+(aq) + Cl-(aq) + Ag+(aq) + NO3-(aq) => Na+(aq) + NO3-(aq) + AgCl(s)

This is where we eliminate the spectator ions. Spectator ions are chemicals or ions that are the same on both sides. It’s sort of like, if you wrote out:

x+5 = y+5

you would know that x=y, right? Because you have the same +5 on either side. The +5 is sitting around doing nothing, and we don’t need it. The rationale with spectator ions is the same. Since NaCl is in solution when we start, it’s already broken up, and same for AgNO3.

Which means we can, for the net ionic, eliminate everything that doesn’t make up the solid AgCl(s).

So the net ionic is:

Cl-(aq) + Ag+(aq) => AgCl(s)

Nice and simple now, isn’t it?

(Now, of course, if you’d started out with solid salt and it ended up dissolved, you’d have to include NaCl(s) as a reactant and Na+(aq) as a product, but that would be because it changed form from the beginning to end. All you have to remember with net ionic equations is that you get rid of what doesn’t change.)

If an electron (mass = 9.11 x 10-31 kg) has an associated wavelength of 7.27 x 10-11 m, what is the electron's velocity? a) 1.66 x 10-9 m/sec b) 1.00 x 107 m/sec c) 1.66 m/sec d) 1.14 x 106 m/sec e) 6.01 m/sec. The answer is B but I have no idea how to get it.

Tue, 11 Dec 2012 18:13:59 -0500

Well, my tortured friend, I’ll have you know that

wavelength = h/(mv)

where h is Planck’s constant (6.63 x 10^-34 Js), m is mass and v is velocity.

So, now we know that:

7.27 x 10^-11 = (6.63x10^-34)/((9.11 x 10^-31)*(v))

6.62 x 10^-41 * v = 6.63x10^-34

v = 10,015,105 or about 1 x 10^7 m/second.

This question is from a thought lab on testing hard and soft water. Please help me. 1 mL of hard water is added to one tube, 1mL of soft water is added to one tube and 1 mL of distilled water to a third. Into each test tube, two drops of 0.1mol/L sodium oxalate solution is added. What will be observed in each test tube and write a chemical equation for each reaction(along with their states) Thank you !!!

Sun, 09 Dec 2012 16:31:38 -0500

Huh! I’ve never had a thought lab, but let’s give this a try.

First off, hard water! According to the internet, it has both Mg2+ and Ca2+ dissolved as ions. Since the sodium oxalate is a solution, that means that the C2O4 2- is the only bit that could take part in a reaction (Na+ will always be soluble and therefore irrelevant).

First off, I know for a fact that MgC2O4 is soluble in water (it doesn’t precipitate, at least in the simple sense) so those ions are boring and useless and do nothing. But the Ca2+ does in fact react, so we can write the equation:

Ca2+ (aq) + C2O4 2- (aq) ==> CaC204 (s)

Fun fact, calcium oxalate is in kidney stones. If it precipitates, it should be white, as far as I can tell.

Now, soft water. Soft water, as far as I can tell, has about the same chemicals in it as hard water, just in much smaller concentrations. This means the above reaction would be valid, although much less likely to happen.

The other option is the sodium oxalate reacting directly with the water, which could be, and this is me guessing, maybe a tiny bit:

Na2C2O4 (aq) + H2O (l) = NaHC2O4 (aq) + NaOH (aq)

NaHC2O4 (aq) + H2O (l) = H2C2O4 (aq) + NaOH (aq)

And the same thing should happen with distilled water, which has no ions in it.

I hope this answer has been at least partway helpful to you! Water is pretty weird.

For the following three sulfur compounds, explain which would be the best oxidizing agent, which would e the best reducing agent, and which is most capable of acting as both: H2SO4, Na2S, S8. Thanks!

Thu, 06 Dec 2012 22:36:15 -0500

My strategy here: figure out the oxidation numbers of the atoms involved. Since none of these compounds have a charge on them, you can assume that the charges of the atoms making them up total up to zero.

First: H2SO4

O has a charge of -2, times 4 = -8,

H has a charge of +1, times 2 = +2,

which means that S has to have a charge of +6 to bring the total to zero.

Second: Na2S

Na has a charge of +1, times 2 = +2,

S, then, has a charge of -2.

Third: S8

The charge on S here is zero, as it is with all compounds made up entirely of one element.

So which one is the best reducing agent? Well, reducing agent means that it’s oxidized in the reaction, and oxidation means electron loss. And in which molecule does S have the most negative charge (most electrons to lose)? Na2S.

Which is the best oxidizing agent, by which I mean it’s reduced in the reaction? Reducing means gaining electrons, and in which molecule does S have the most positive charge (least number of electrons)? H2SO4.

Lastly, S8 has a charge of zero, and so is the most capable of either gaining electrons (acting as an oxidizing agent) or losing electrons (acting as a reducing agent).

Boric Acid solution is used as an eyewash. What mass of boric acid is present in 250 g of solution that is 2.25%(m/m) acid in water? Thank you.

Mon, 03 Dec 2012 20:44:33 -0500

I’m presuming ‘2.25%(m/m)’ means that boric acid makes up 2.25% of the eyewash, which makes this easy-breezy (is that a phrase?).

A percentage is exactly what it sounds like - part of a hundred. So, numerically, 2.25% = .0225, and to find the mass of boric acid in water, all you have to do is multiply .0225 x 250 g = 5.625 grams of boric acid.

In a reaction 3.84 moles of carbon is reacting with 1.68 moles of sulfur dioxide. What is the limiting reagent? Excess reagent? how many moles of carbon disulfide can form? How many moles of the excess are left?

Mon, 03 Dec 2012 20:42:05 -0500

Oh, stoichiometry, strange picture of Tom Hiddleston laughing, is a lot of fun, if you know what you’re doing.

This question is all about mole ratios. First:

The equation of the reaction.

5C + 2SO2 = CS2 + 4CO

(Once you said it produced carbon disulfide, I worked from there, but I assume you’re familiar with balancing chemical equations.)

You have 3.84 moles C and 1.68 moles SO2.

Limiting and excess reagent are just a matter of figuring out which of those moles are used up first, and that’s where mole ratios come in. We have 3.84 moles of C, and that’s in a 2/5 ratio with SO2 in the reaction. Using that ratio we can figure out how many moles of SO2 it would take for 3.84 moles of C to be used up completely, hence:

3.84 moles C (2 moles SO2 / 5 moles C) = 1.536 moles for a complete reaction, which means that even after the 3.84 moles of C have been used up, there’s still a whole .144 moles of SO2 left to react. In this case, carbon is the limiting reagent and sulfur dioxide is the excess reagent.

We can also do this the other way: 1.68 moles SO2 (5 moles C / 2 moles SO2) = 4.2 moles of carbon required to completely react that SO2, which means that the SO2 is still in excess.

As for how many moles of carbon disulfide, that’s the same logic. Just make sure to use the number of moles of carbon rather than the moles of sulfur dioxide, because there’s more sulfur dioxide than can react to form carbon disulfide.

3.84 moles C (1 mole CS2 / 5 moles C) = .768 moles of carbon disulfide.

Calculate the mass of 3.62 x 10^24 molecules of glucose? :) thanks!

Thu, 29 Nov 2012 23:57:27 -0500

Well, there are 6.022 x 10^23 atoms in a mole. So,

3.62 x 10^24 / 6.022 x 10^23 = 6.011 moles

The equation for glucose is C6H12O6, with a molar mass of 180, so:

6.011 x 180 = 1,081 grams.

How many grams of air are required for an automobile to travel from Bay to Falls. This is a distance of 670 km. Assume the following: a) Gasoline is a pure octane C8H18 b) The average fuel consumption is 10L per 100 km. c)Air has a density of 1.21g/L d) the density of gasoline is 0.703g/mL. The balanced chemical equation for the complete combustion of octane is 2C8H18 + 25O2---> 16CO2 + 18H20 THANK YOU

Thu, 29 Nov 2012 00:20:11 -0500

You know the mole ratio, so the first step from here is to figure out how many liters of gasoline you need to make that trip.

670 km (10 L / 100 km) = 67 L

To use mole ratios, we need moles. So from liters, we need to find grams.

6700 L (1000 mL / 1 L) = 67,000 mL

6,700,000 mL (.703 grams / 1 mL) = 47,101 grams C8H18

47,101 grams (1 mole C8H18 / 114 grams C8H18) = 413.16 moles of C8H18.

Using the mole ratio from the problem, we know the mole ratio from C8H18 to O2 is 25/2, so:

413.16 moles C8H18 (25 m O2 / 2 m C8H18) = 5,164.58 moles O2

5,164.58 moles O2 (32 grams O2 / 1 mole O2) = 165,266.66 grams of O2 = 165.26 kg O2

Now, you haven’t told me what percentage of air oxygen is, but a cursory glance at the internet suggest it to be around 23.3%.

So 165.26/.233 = 709.3 kg, all the air necessary for one journey from Bay to Falls. Happy travels.

Hey! I've got a lab to do that involves finding some sort of chemical equation that reduces Fe(III) to Fe(II). Help? It can involve any other chemicals. It just needs to reduce Fe(III). Thanks!

Mon, 19 Nov 2012 23:07:00 -0500

Sure thing!

All Fe (III) means is that it’s Fe with a charge of 3+. To reduce means gaining electrons, so:

Fe3+ + e- ==> Fe2+

All you have to do is throw an electron in there. It’s happy to stick around.

This blog is amazing. I don't understand the mole concept at all. Do you know any good websites to help me? I've seen a few, but I don't understand. Thank you!

Thu, 15 Nov 2012 20:08:32 -0500

The mole is a convenient way for Chemists to measure elements. You know how you use the term ‘dozen’ to mean twelve? And a gross is 144, a couple is two (unless you’re sort of uppity and like to argue that kind of thing). A ‘mole’ is a number.

A mole, as it turns out, is 6.022 x 10^23. It’s a big number. But on it’s own, it doesn’t mean anything.

A mole, in the chemistry sense, is used to measure atomic masses. Let’s say I have 6.022 x 10^23 atoms of Oxygen. Those atoms, all together, are going to weigh about sixteen grams. So, you say, ‘a mol of Oxygen is 16 g’, much in the way you’d say ‘this bakers dozen of cream puffs weighs half a pound’ (I know nothing about the weight of pastry) but, say, a baker’s dozen of dense little chocolate cakes might weigh a pound and a half.

A mol is just a number, and a way for chemists to say, ‘okay, when you have this big number of atoms of this type of element, it weighs this much’. No one, however, has weighed in on the pressing matter of pastry weight.

For a better explanation from a less hungry individual, I’d recommend this.