Given Grams, Convert to Moles

Return to Mole Table of Contents

In chemistry, the mole is the standard measurement of amount. However, balances DO NOT give readings in moles. Balances give readings in grams.

So the problem is that, while we compare amounts of one substance to another using moles, we must also use grams, since this is the information we get from balances.

There are three steps to converting grams of a substance to moles.

- Determine how many grams are given in the problem.
- Calculate the molar mass of the substance.
- Divide step one by step two.

The three steps above can be expressed in the following proportion:

In this particular lesson, the moles of the substance (lower left) will be the unknown (which will be signified by the letter x). The exact same proportion is used in the moles-to-grams conversion lesson. Then the "x" will reside in the upper left.

This proportion is a symbolic equation. When you solve a particular problem, you insert the proper numbers & units into the proper places of the symbolic equation and then you solve using cross-multiplication and division. Also, do not attach units to the unknown. Let it be simply the letter "x." The proper unit should evolve naturally from solving the proportion and cancellation of units.

Make sure you have a periodic table and a calculator handy.

Problem #1 - Convert 25.0 grams of KMnO_{4} to moles.

Step One: The problem will tell you how many grams are present. Look for the unit of grams. The number immediately preceeding it will be how many grams. Common abbreviations for grams include g (just the letter) and gm.

I suppose that a problem can be worded in such a way that the number of grams comes after the unit, but that type of trickery isn't very common in high school.

The problem gives us 25.0 grams.

Step Two: You need to know the molar mass of the substance. Please refer to the lessons about calculating the molecular weight and molar mass of a substance if you are not sure how to calculate a molar mass.

The molar mass of KMnO_{4} is 158.034 grams/mole. Please take a moment and calculate the molar mass of KMnO_{4}, just to be sure.

Step Three: You divide the grams given by the substance's molar mass:

The answer of 0.158 mole has been rounded to three significant figures because the 25.0 value had the least number of significant figures in the problem.

If this problem were set up like the proportion above, you would have this:

Then, cross-multiply and divide to solve for the unknown.

Problem #2 - calculate how many moles are in 17.0 grams of H_{2}O_{2}

17.0 grams are given in the text of the problem. The molar mass is 34.0146 grams/mole, which is calculated using the formula and the atomic weights on a periodic table. You may want to calculate this value as a double-check of your mastery of calculating molar masses.

This answer has been rounded to three significant figures because of the 17.0.

This problem can also be set up as a proportion:

Calculate the moles present in:

1) 2.00 grams of H_{2}O

2) 75.57 grams of KBr

3) 100. grams of KClO_{4}

4) 8.76 grams of NaOH

5) 0.750 grams of Na_{2}CO_{3}