[ChemCatalyst]

To convert from grams to moles, would you divide? Sorry that my first question is a little vague. Also, I am not sure how to convert from moles back to grams. Let me know if more information is needed in order to help me out.

[Capuchin]

One mole of a substance has a mass of (the molecular/atomic mass) grams

So for instance, Carbon has an atomic mass of 12, so one mole of carbon weighs 12 grams.

[rudi_in]

Thank you for posting your question to the Ask Me Help Desk.

In order to convert from grams to moles...

Start with the mass of the given and

multiply by (1 mol given) / (molar mass of the given)


For example...

Convert 4.75 g C to moles of C

4.75 g C x 1 mol C / 12.01g C

Simplify to get...

0.40 mol


To convert moles to grams...

multiply by (molar mass of the given) / (1 mol given)


For example...

Convert 3.12 mol Na to grams Na

3.12 mol Na x (22.99g Na) / (1 mol Na)

Simplify to get...

71.73g Na

[ChemCatalyst]

Now that I understand how to do those things, I need help figuring out how to solve stoichiometry problems. I have several worksheets that try to explain it, but none of them really explain it in a way I can understand. There is a fence method, but I always get confused as to which numbers and ions and what not go in certain spots.

[rudi_in]

If you can do these problems, stoichiometery should not be much of a problem as you are essentially performing the same math.

In this stage of the course you are most likely dealing with the 4 basic types of stoichiometry problems

1. Mole-Mole (given moles and converting to moles) 1 conversion factor

2. Mole-Mass (given moles and converting to mass) 2 conversion factors

3. Mass-Mole (given mass and converting to moles) 2 conversion factors

4. Mass-Mass (given mass and converting to mass) 3 conversion factors


In each of these cases you MUST have a correctly written, balanced equation in order to solve correctly.

Each type of conversion involves the use of one or more conversion factors. The number of conversion factors needed for each type is indicated above. (This is the way I teach it.)

In each case you must begin with the given value...



For number 1: Mole-Mole

(moles of given) x ((moles needed / moles given))

The information in the red parentheses comes from the balanced equation. You will use the coefficients in front of the needed (what the problem is asking for) and the given to fill in here. This is called the molar ratio. It will always be a whole number.

Example:

2H2 + O2 --> 2H2O

Given 8.3 moles hydrogen, how many moles of water can be produced?

8.3 mol H2 x (2 mol H2O / 2 mol H2)

Notice the H2 will cancel leaving the H2O. Moles needed goes on top. What the question is asking for goes on top. The question asks how much water so water is needed and it goes on top. We are given a value for H2 so that goes on the bottom. This information comes from the balanced equation only. In this reaction, there is always a ratio of 2 mol H2 to 2 moles of water.

Answer: 8.3 mol H2O



2. Mole-Mass

Use the same setup as above but add one more conversion factor. Convert to mass just like we did in your other question.

Imagine the above example altered slightly...

Given 8.3 moles hydrogen, what mass of water can be produced?

(moles of given) x (moles needed / moles given) x ((molar mass of needed / 1 mol needed))

8.3 mol H2 x (2 mol H2O / 2 mol H2) x (18.02g H2O / 1 mol H2O)

Answer: 149.57g H2O



3. Mass-Mole

Go in the reverse order...

How many moles of H2 are needed to produce 149.57g H2O?

mass of given x (1 mol given / molar mass of given) x (moles needed / moles given)

149.57g H2O x (1 mol H2O / 18.02g H2O) x (2 mol H2 / 2 mol H2O)

Notice the conversion factors are flipped from the other example. That is because the needed and given are different now but the rules stay essentially the same.

Answer: 8.3 mol H2



4. Mass-Mass

Take the last example and add one more conversion factor...

What mass of H2 is needed to produce 149.57g H2O?

mass of given x (1 mol given / molar mass of given) x (moles needed / moles given) x ((molar mass of needed / 1 mol needed))

149.57g H2O x (1 mol H2O / 18.02g H2O) x (2 mol H2 / 2 mol H2O) x ((2.02g H2 / 1 mol H2))

Answer: 16.77g H2



The fence is just a fancy way of setting up the conversion factors since we are just multiplying a bunch of fractions...

|-|x|-|x|-|x|-|

Stick with the layout rules I gave you and you should be OK.

Let me know if you need anything else.

Good Luck!

[sarahjt1]

Can you explain your equation in #2? I'm also learning this stuff but I don't understand how you know WHAT to plug in WHERE... :)

"(moles needed / moles given)" How do you know what to plug into these values? Where did you get 2 mol H2O and 2 mol H2?


Given 8.3 moles hydrogen, what mass of water can be produced?

(moles of given) x (moles needed / moles given) x ((molar mass of needed / 1 mol needed))

8.3 mol H2 x (2 mol H2O / 2 mol H2) x (18.02g H2O / 1 mol H2O)

Answer: 149.57g H2O

[sarahjt1]

Never mind! Is it because you already balanced the equation but just didn't show that step... got it! :)