[thinkpink654323]

I'm having a lot of trouble with this stuff... im only a middle school student
how do you convert moles to grams, grams to moles, and do more complex equations involving these concepts... please respond as soon as possible.. my test is just days away!
:confused: I'm quite confused about the entire concept of lines of equality as well

[rudi_in]

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

Here is a brief review with some steps to follow when solving these conversions.

There are 4 basic types of conversions you will most likely be working with.

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. In subsequent examples the red parentheses indicate new steps.

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

I am going to guess that your teacher calls the line between the top and bottom values within the conversion factor a line of equality. In other words, the top value is equal to the bottom value.

For example 20 nickels/1 dollar

Both have an equal value even though they look different. Just as 5/5 equals 1 - so does 20 nickels/1 dollar.

When we multiply a number by 1 we do not change it's value. It allows us to express it in a different way with different units.

Let me know if you need anything else.

Good Luck!

[evanska567]

i'm having a lot of trouble with this stuff...im only a middle school student
how do you convert moles to grams, grams to moles, and do more complex equations involving these concepts...please respond as soon as possible..my test is just days away!!
:confused: im quite confused about the entire concept of lines of equality as well

To convert back and forth, just change the unit from mol to gram OR gram to mol

[rudi_in]

Two things,

First of all, the original post is about 6 months old... and has been answered already.

Secondly, the information you provide is inaccurate and misleading to any who may choose to read this newly bumped post.