PDA

View Full Version : Stoichiometry Help

Agent Intellect
24th-March-2010, 03:57 PM
Being as inept at math as I am, having reached stoichiometry in my chemistry class has been a bit frustrating for me. If anyone knows some good recourses for learning about it, I'd appreciate if they could point me there. And, if anyone is willing to help me with a few problems (I'm sure there has to be someone here that enjoys it) I could use some assistance on the following problems:

1. Calcium tablets are available in varying chemical forms as a dietary supplement. Compare the mass of the varying forms of tablets required to furnish 500 mg of Ca +2 cations.
a. CaCO3
b. Ca3(C6H5O7)2
c. Ca(C3H5O3)2

2. Explain why it takes 8 grams of Oxygen per gram of Hydrogen to form water.
2H2 + O2 -> 2H2O

3. How much Oxygen must you inhale to consume 10.0 grams of the sucrose in cane sugar?
C12H22O11 + 12O2 -> 12CO2 + 11H2O

EditorOne
24th-March-2010, 04:07 PM
Sorry, stoichiometry hadn't been invented when I took chemistry. We were still trying to understand "fire."

Da Blob
24th-March-2010, 04:34 PM
I am sure it is just a temporary mental block on your part. These questions are just dealing with atomic weights (per periodic table). Unfortunately i do not have a conversion table to answer those questions for you. All I remember is how to deal with quantities of moles....

This chemlin.net site looks useful, but i 'll look for more - there is a online help source available btw

http://www.chemlin.net/chemistry/stoichiometry.htm

Kassie
24th-March-2010, 04:41 PM
I've been having the same problem lately. I'm pretty good at math, but the chemistry teacher at our school can't explain anything right. He can't even keep control of the class most of the time. Plus, I can't switch to a different teacher, because he's the only chemistry teacher. None of us get it, and he keeps giving us silly little quizzes over this stuff after explaining it (very poorly) once, and expecting us to know everything.:(
It's so frustrating. Good thing the quizzes aren't worth much grade-wise.

24th-March-2010, 05:04 PM
1. Calcium tablets are available in varying chemical forms as a dietary supplement. Compare the mass of the varying forms of tablets required to furnish 500 mg of Ca +2 cations.
a. CaCO3
b. Ca3(C6H5O7)2
c. Ca(C3H5O3)2

2. Explain why it takes 8 grams of Oxygen per gram of Hydrogen to form water.
2H2 + O2 -> 2H2O

It's all about molecular weight here. H =1g, O = 16g (roughy). So in every H2O molecule you have two H, and 1 O. So if a mole of H2O was broken down you would have 2 grams of H and 16 grams of O. A ratio of 1:8 in grams.

However if you were using moles you'd have 2 moles of H and 1 mole O. These numbers are the molecular coefficients from the the empirical equation above.

3. How much Oxygen must you inhale to consume 10.0 grams of the sucrose in cane sugar?
C12H22O11 + 12O2 -> 12CO2 + 11H2O

It's another molecular weight/ molar mass question.
The molecular weight of sucrose is 342.3 g, so if you inhale 12 moles (molecular coefficient) of O2 or 383.88 g you will inhale one mole or 342.3 g of sucrose.

So how many moles is 10 g of sugar?
10g/341.3g = 0.0292 Moles * molecular coefficient (12) = moles of O2 = 0.351 * the molar mass (remember O2 would have a molecular weight of 32, not 16 like O) = grams of O2 or 11.25g

sidenotes:
* = multiply

My calculations were rough and may not be precise enough for an answer.

With question 3, I assumed they were asking about a direct relationship with oxygen (O2), but if the were referring to the right side of the chemical equation with a vague term like 'air' 12CO2 + 11H2O (even though that's not the composition of air >.>), I would be wrong.

Da Blob
24th-March-2010, 05:22 PM
These are simple problems, but I am lost without a CRC handbook when it comes to Chemistry. My quick perusal of the internet did not seem to help much. It seems as though everyone talks about T-diagrams...

http://misterguch.brinkster.net/molecalculations.html

Any way for the first problem, 500 grams of Ca divided by 40.078 (mole weight of calcium?) will give the number of moles required. Then it is just a matter of multiplying the mole weights of the various chemical compounds listed by that number (approx. 12.5)

24th-March-2010, 05:31 PM
Standard periodic table (http://www.ptable.com/)should give you all the molecular weights you need.

T-Tables always annoyed the hell out of me, they seem to make things unnecessarily confusing.

ashitaria
24th-March-2010, 07:41 PM
Try reading the textbook. The textbook always has examples on how to do it. Anyway, when my teacher was teaching stoicmetry...I wasn't paying attention anyway.

Learned how to do everything by the textbook....and hour before the test! :evil:

Agent Intellect
24th-March-2010, 07:42 PM
Well, I was able to do the problems myself, but thanks for the input.

The main problem I had was that I was overthinking them. For some reason I have a mind that wants to be able to figure everything out via "thought experiment" by picturing it in my head and I end up throwing all the math out the window and sitting there trying to tackle something in this "thought experiment" type method - it's served me well when writing and even for biology, but not so much for chemistry, math, and physics.

--------

For number 1 I simply got the MM of the compounds and converted to mg (since that's the unit they wanted it in) then did

100090 mg CaCO3 / 40080 mg Ca2+ = X CaCO3 / 500 mg Ca2+

Cross multiplied and got divided by 40080 to get 1248.628 mg CaCO3

Which can be checked by doing 500 mg Ca2+ / 1248 mg CaCO3 = ~.40
And 40.08 g/mol Ca / 100.09 g/mol CaCO3 = ~.40

And I applied the same method for the other two.

---------

Number two I did exactly what Nyx said.

---------

For 3 I pretty much did the same as number 1, and did the proportions.

342.34 g C12H22O11 / 384 g O2 = 10 g / X g

Got 342.34X / 342.34 = 3840 / 342.34

Which gave me X = 11.22 g O2

EDIT: btw Nyx, that is an awesome periodic table.

Marbas
24th-March-2010, 11:58 PM
Standard periodic table (http://www.ptable.com/)should give you all the molecular weights you need.

T-Tables always annoyed the hell out of me, they seem to make things unnecessarily confusing.

T-Tables always struck me as completely unnecessary wastes of space. They never confused me, they just seemed absolutely pointless. And messy to boot.

HecticRat
29th-March-2010, 07:08 PM
I've never had an easy time with chemistry, probably due in large part to the fact that I've never once had a good teacher (one was half decent), but stoichiometry was never that difficult as far as I remember. Isn't it almost always just a matter of converting mass A to mols A, mols A to mols B, and then mols B to mass B?

Architectonic
1st-April-2010, 02:36 PM
You guys are studying the following sort of stuff yeah?

http://ibchem.com/IB/ibsyllabus-2009.htm
http://www.revision-notes.co.uk/IB/Chemistry/

Looks straightforward, though I've no doubt forgotten some of it as I''ve reformatted my brain a few times since then.

Agent Intellect
2nd-April-2010, 03:35 AM
I just had the stoichiometry test today; it was really quite easy after I sat down and actually studied the math instead of taking my usual "intuitive" approach. We haven't done anything with the ideal gas law or osmotic pressure or molality or any of the other bullshit I saw while skimming through my book.

I did see this today though:

Thermodynamics of Hell

First, we need to know how the mass of Hell is changing in time. So we need to know the rate that souls are moving into Hell and the rate they are leaving. I think that we can safely assume that once a soul gets to Hell, it will not leave. Therefore, no souls are leaving. As for how many souls are entering Hell, lets look at the different religions that exist in the world today. Some of these religions state that if you are not a member of their religion, you will go to Hell.
Since there are more than one of these religions and since people do not belong to more than one religion, we can project that all souls go to Hell.

With birth and death rates as they are, we can expect the number of souls in Hell to increase exponentially.

Now, we look at the rate of change of the volume in Hell because Boyle's Law states that in order for the temperature and pressure in Hell to stay the same, the volume of Hell has to expand proportionately as souls are added.

This leaves two possibilities:

1. If Hell is expanding at a slower rate than the rate at which souls enter Hell, then the temperature and pressure in Hell will increase until All Hell Breaks Loose.

2. If Hell is expanding at a rate faster than the increase of souls in Hell, then the temperature and pressure will drop until Hell Freezes Over.

So which is it? If we accept the postulate given to me by Teresa during my Freshman year,"... it will be a cold day in Hell before I sleep with you," and take into account the fact that I still have not succeeded in sleeping with her, then #2 cannot be true, and thus I am sure that Hell is Exothermic and will not freeze.

Architectonic
2nd-April-2010, 07:25 AM
With these simple algebraic functions, you don't even have to remember formulas. You can often just look at the units, divide or multiply the variables (sometimes including squaring/cubing things like radii) you are given until you get the correct units for the answer. You should always make sure that the units add/cancel correctly when doing problems as a way of double checking you are using the right formula. (note I'm talking about high school science..)

When you get to slightly harder stuff involving trigonometric, logarithms, exponentials etc, these are usually derived though calculus. If you have a reasonable understanding of calculus, it is often much easier (or intuitive at least) to use calculus than to remember many formulas. (Thermodynamics, Electromagnetism etc are much friendlier in differential form..)

Agent Intellect
6th-April-2010, 09:00 PM
So, how about thermochemistry help (if anyone is interested)?

1.) The ΔH per mole of CaCl2 = -81.0 kJ. Calculate the final temperature of an aqueous solution of 30 grams of CaCl2 is added to 100 mL of water at 23 degrees Celsius. Assume the specific heat of the solution is the same as that of water (4.184 J/g*C)

2.) 50 grams of NH4Cl was dissolved in 100 mL of water. The calculated ΔH for the reaction was +15.2 kJ. What is the ΔT obtained in this experiment?

3.) A student prepares a cold pack by dissolving 30 grams of ammonium nitrate in 100 grams of water at 20 degrees C. If the temperature drops from 20 degrees C to 0 degrees C, what is the calculated experimental value for ΔH per mole of ammonium nitrate?

------------------

I'm assuming this formula:

q = m*Cp*ΔT

q = heat absorbed by the water in joules (J)
m = mass of substance
ΔT = tfinal - tinitial
Cp = specific heat of water = 4.184 J/g*C

Is required here, but I've approached these problems every way I could using it, but all I get is nonsense sounding answers (bearing in mind I have no access to any of the actual answers to these). The bullshit thing is that this is stuff I need for my lab tonight but we haven't even started on it in lecture (just had test on our last unit last thursday).

Agent Intellect
7th-April-2010, 11:32 AM
Well, for anyone that cares (and perhaps it's a good thing you don't) I figured out what I was doing wrong. First off, I was doing things right except that 1) I wasn't converting kJ to J and 2) some of my nonsense answers weren't actually nonsense, they only looked like it to me (I was expecting much smaller changes in temperature). Math has always made me lose my patience though...