How to Convert Grams to Moles and Vice Versa

Here's how to make mole-to-gram and gram-to-mole conversions

Compounds are weighed using scales to yield grams. It's often necessary to convert grams to moles for chemistry calculations.
Compounds are weighed using scales to yield grams. It's often necessary to convert grams to moles for chemistry calculations. Peter Muller / Getty Images

This worked example problem shows how to convert the number of grams of a molecule to the number of moles of the molecule. Why would you need to do that? This type of conversion problem mainly arises when you are given (or must measure) the mass of a sample in grams and then need to work a ratio or balanced equation problem that requires moles.

Key Takeaways: Converting Moles to Grams (and Vice Versa)

  • Grams and moles are two units to express the amount of matter in a sample. There is no "conversion formula" between the two units. Instead, you must use atomic mass values and the chemical formula to do the conversion.
  • To do this, look up atomic masses on the periodic table and use the formula mass to know how many atoms of each element are in a compound.
  • Remember, subscripts in a formula indicate number of atoms. If there is no subscript, it means there is only one atom of that element in the formula.
  • Multiply the number of atoms of an element by its atomic mass. Do this for all the atoms and add the values together to get the number of grams per mole. This is your conversion factor.

Grams to Moles Conversion Problem

Determine the number of moles of CO2 in 454 grams of CO2.

Solution

First, look up the atomic masses for carbon and oxygen from the periodic table. The atomic mass of C is 12.01, and the atomic mass of O is 16.00. The formula mass of CO2 is:

12.01 + 2(16.00) = 44.01

Thus, one mole of CO2 weighs 44.01 grams. This relation provides a conversion factor to go from grams to moles. Using the factor 1 mol/44.01 g:

moles CO2 = 454 g x 1 mol/44.01 g = 10.3 moles

Answer

There are 10.3 moles of CO2 in 454 grams of CO2.

Moles to Grams Example Problem

On the other hand, sometimes you're given a value in moles and need to convert it to grams. To do this, first calculate the molar mass of a sample. Then, multiply it by the number of moles to get an answer in grams:

grams of sample = (molar mass) x (moles)

For example, find the number of grams in 0.700 moles of hydrogen peroxide, H2O2.

Calculate the molar mass by multiplying the number of atoms of each element in the compound (its subscript) times the atomic mass of the element from the periodic table.

Molar mass = (2 x 1.008) + (2 x 15.999) Note the use of more significant figures for oxygen
Molar mass = 34.016 grams/mol

Multiply the molar mass by the number of moles to get the grams:

grams of hydrogen peroxide = (34.016 grams/mol) x (0.700 mol) = 23.811 grams

There are 23.811 grams of hydrogen peroxide in 0.700 moles of hydrogen peroxide.

Performing Grams and Moles Conversions

Here are some tips for performing these conversions:

  • The two problems most commonly encountered are setting up the problem incorrectly, so the units don't cancel out and give the correct result. It helps to write out the conversion and make sure units cancel. You may want to draw a line through them in complex calculations to keep track of active units.
  • Watch your significant figures. Chemistry professors are unforgiving when it comes to reporting an answer, even if you set up the problem correctly.

Moles to Grams Conversion Problem

Sometimes you are given moles and need to convert it into grams. This worked example problem shows you how to convert moles to grams.

Problem

Determine the mass in grams of 3.60 mol of H2SO4.

Solution

First, look up the atomic masses for hydrogen, sulfur, and oxygen from the periodic table. The atomic mass is 1.008 for H, 32.06 for S, and 16.00 for O. The formula mass of H2SO4 is:

2(1.008) + 32.06 + 4(16.00) = 98.08

Thus, one mole of H2SO4 weighs 98.08 grams. This relation provides a conversion factor to go from grams to moles. Using the factor 98.08 g / 1 mol:

grams H2SO4 = 3.60 mol x 98.08 g / 1 mol = 353 g H2SO4

Answer

There are 353 grams of H2SO4 in 3.60 moles of H2SO4.