Stoichiometry, the bane of my existence.

[Dovahkiin]

So my professor sucks. I feel bad about saying it but I´m like almost to the end of my course and I still understand nothing of Stoichiometry.

 

I have a lab of just Stoich questions and I am really struggling. She gave the answers but I do not want the answers, I want to know how to do them.

 

Anyway at the moment here is the question I am stuck on (before going on to the next ones)

 

I´ve already balanced the equation.

 

Al(OH)3 + 3HCL --- AlCl3 + 3H2O

 

c. What mass of water (H2O) is produced by the reaction of 10 grams of AL(OH)3?

 

All my attempts at anything have been wrong.

 

I´ve done random stuff like

 

10g AL(OH)3 = .128 moles AL(OH)3

H2O = 18.02molecular mass

 

Anyone help please?

[pavelcherepan]

OK, you've balanced the reaction and it's all good. But these coefficients in front of compounds are for ideal-case scenario when 1 mol of aluminium hydroxide reacts with 3 moles of hydrochloric acid. But what you have is 0.128 moles of Al(OH)₃ so then you multiply this number by stoichiometric coefficients for each of the compounds and you will find the actual number of moles of H₂O.

 

Then you multiply this actual number of moles by molecular weight that you already know and the result will be the number of grams of H₂O.

Edited April 16, 2015 by pavelcherepan

[hypervalent_iodine]

What pavelcheperpan has said is correct for your question, though I would clarify that you multiply only by the coefficient of the product you want, rather than all of them.

 

The general process is to convert the mass or volume, etc., of what you have into moles, multiply that number by the ratio (the coefficient of the thing you want / the thing you have; in your case 3/1) to get the number of moles of your product or whatever it is the question is asking for and then convert that number into whatever units are required.

[studiot]

Wouldn't it be nice if every molecule and atom had the same mass and weight?

 

But they don't, any more the every person has the same size of show or hat.

 

The equation of the chemical reaction tells you how many molecules of one substance react with how many molecules of another to produce how many molecules of product.

 

So your equation says (get used to saying it to yourself)

 

One molecule of aluminium hydroxide reacts with three molecules of hydrogen chloride to produce one molecule of aluminium chloride and three molecules of water.

 

Now all you need are the molecular weights.

 

You get these by adding the atomic weights of the consitiuent atoms.

 

You get the atomic weights by looking them up in a table, so:-

 

Edit as fuzzwood pointed out I looked in the wrong table (atomic numbers)

A serious mistake

I have now corrected.

 

Hydrogen is 1

Aluminium is 13 27

Chlorine is is 17 35.5

Oxygen is 16

 

Now you are motoring.

 

The molecular weights are

 

Aluminium hydroxide is one aluminium plus 3 oxygens plus 3 hydrogens = 13 + (3x16) + (3x1) = 13 + 48 + 3 = 64

27 + (3x16) + (3x1) = 27 + 48 + 3 = 78

Hydrogen chloride is 1 + 17 = 18 1 + 35.5 = 36.5

Aluminium chloride is 13 + (3x 17) = 64 27 + (3x35.5) = 133.5

Water is 2 + 16 = 18

 

Now I have not said what the units of molecular weights are.

That is because we can work in any weight units we like so long as we use the same ones throughout.

 

Common metric units of weight are

 

micrograms, milligrams, grams, kilograms, tonnes

 

Any any will do - different folks use different ones so doctors use micro and milligrams, physicists use grams, bakers use kilograms and cement manufacturers use tonnes.

 

Chemists like to work in grams and even coined the term gram-molecule whichis just the molecular weight in grams. This was shortened to mole.

 

But kilogram-molecules or microgram-molecules could also be used just as well.

 

So back to your stochiometric equation, and using grams this says

 

one gram-molecule of aluminium hydroxide reacts with three gram-molecules of hydrogen chloride to produce one gram-molecule of aluminium chloride and three gram-molecules of water.

 

or as pavel had it

 

One mole of aluminium hydroxide reacts with three moles of hydrogen chloride to produce one mole of aluminium chloride and three moles of water.

 

In numbers

 

 

(1x78) grams of aluminium hydroxide reacts with (3x36.5)grams of hydrogen chloride to produce (1x133.5) grams moles of aluminium chloride and (3x18)grams of water.

 

78grams of aluminium hydroxide reacts with 109.5grams of hydrogen chloride to produce 133.5grams of aluminium chloride and 54grams of water.

 

Notice as a check that the total weight on one side of the equation = the total weight on the other side

 

78 + 109.5 = 187.5 = 133.5 + 54.

 

This is always true of weights, but the number of molecules may be different.

 

So to complete your question

 

(78/78)grams of aluminium hydroxide reacts with (109.5/78)grams of hydrogen chloride to produce (133.5/78)grams of aluminium chloride and (54/78)grams of water.

1gram of aluminium hydroxide reacts with (109.5/78)grams of hydrogen chloride to produce (133.5/78)grams of aluminium chloride and (54/78)grams of water.

 

10grams of aluminium hydroxide reacts with (10x48/64)grams of hydrogen chloride to produce (10x133.5/78)grams of aluminium chloride and (10x54/78)grams of water.

 

Finally we usually talk about molecular masses, rather than molecular weights these days. They are the same thing to all intents and purposes.

 

Does this help?

Edited April 17, 2015 by studiot

[hypervalent_iodine]

Again, to clarify some things.

 

I know studiot has said that gram-molecule is the same as saying mole, but the term we use in chemistry is always moles. It is best to stick to that.

 

Since this is fresh material for you, I would again recommend sticking to a step-wise approach. If you have a process that you use for every question of this type that is unambiguous and that you understand, you will not have a problem answering these questions. You have been given a number of the-same-but-different approaches here, however if you are more of a visual person, I came across this diagram (I've had to draw it myself, but you get the idea) in a student's text book a few months ago that I quite liked. It more or less summarises what I said in my last post.

The above is just an example. If the units of A were something else, substitute the first arrow with the appropriate equation to convert to moles. And of course, the method could be applied to any combination of A, B, C and D; it depends on what your question is.

Personally, and I do not mean this to disparage studiot, I think you are considerably better of not using his method. It will give you the correct answer, but changing the coefficients in your reactions from moles (as given in the reaction - 1 mole of Al(OH)₃ reacts to make 3 moles of water) to grams is a round-about and confusing way to answer your question. It will also be unhelpfully complicated for you if the units you are dealing with are, say volume instead of grams. Stick to moles and convert to your desired units at the end. It's much simpler and more than likely, much more compatible with how your teacher will be going through it.

 

Finally, to reiterate what I said before, if your question is only asking you to account for the amount of one of your products or reactants given the only one other component of your reaction, it is not necessary to calculate everything. It is certainly good practice while you are learning and as studiot rightly points out, a good way to check your results. If you are confident in your process, then doing this may only serve to increase the amount of time you are taking to answer questions in an exam and clutter your exam sheet in the case of short answer questions.*

 

 

 

*I have mentioned this to pavel already, but I also wanted to mention quickly that I am only including this clarification because my experience is that showing students how to answer a questions with a load calculations that aren't strictly in answer to it can sometimes be confusing for them. It is probably an unnecessary comment to have to make, I am not sure, though I think it is best to be safe and as clear as possible as to how our explanations fit in with the OP.

[studiot]

I stand by my method, but this thread is not the place to discuss teaching methods, except to say that it has achieved great success preventing medical drug errors for medical practioners, thereby saving lives.

 

I would be happy to discuss further in another thread.

Edited April 17, 2015 by studiot

[hypervalent_iodine]

I stand by my method, but this thread is not the place to discuss teaching methods, except to say that it has achieved great success preventing medical drug errors for medical practioners, thereby saving lives.

 

I would be happy to discuss further in another thread.

 

Look, to each his or her own. If the OP finds it better to work that way, more power to both of you. I'm not even trying to say that your method is bad, I'm simply saying that it is unlikely to be similar to how the OP has been going through questions in class and is thus could lead to confusion for them. No high school / first year / any chemistry class (that I have encountered) will teach you to change the coefficients in a reaction from moles to grams. That simply doesn't make sense in that context.

[Fuzzwood]

Also, the molar mass of chloride is 35.45, not the earlier mentioned 17.

[imatfaal]

....

 

You get the atomic weights by looking them up in a table, so:-

 

Hydrogen is 1

Aluminium is 13

Chlorine is is 17

Oxygen is 16

 

...

 

 

Surely these are numbers of protons - ie atomic numbers not atomic weights

Apart from oxygen - which is neither

[studiot]

Thank you fuzzwood +1 I have now put my post right.