Name: ________________________________________
Date: _______________
Gram Formula Mass
If a scientist wanted to produce water in the laboratory they would need 2 atoms of hydrogen for every
1 atom of oxygen. How can a scientist measure out exact quantities of atoms? THEY ARE UNABLE TO DO THIS
because a single atom cannot be seen or measured directly. We are able to however take measurements of
mass and volume. By using the quantity known as the mole, scientists are able to link the number of atoms or
molecules (particles) of any substance to a macroscopic amount. One mole is also equal to 6.02 x 1023 particles
of a substance, 22.4 liters of a gas AND the gram-formula mass of a substance.
Since we are able to take measurement of mass and one mole equals the gram formula mass (gfm) for
a substance, we can now measure out an amount in grams and convert it to the exact number of molecules or
atoms.
According to the periodic table, the atomic mass of hydrogen is 1.0 amu and oxygen is 16.0 amu but
again, we are unable to directly measure atoms because of their size. Instead of using atomic mass units, we
will now use a unit of grams. We now assume the mass of hydrogen is 1.0 gram and the mass of oxygen is 16.0
grams. To generate water, the following balanced reaction between hydrogen and oxygen gas occurs:
2 H2 + O2 → 2 H2O
4.0 g 32.0 g 36.0 g (Mass is conserved)
A scientist would need to measure out 4.0 g of hydrogen gas (the mass of 2 moles of hydrogen) and 32.0 g of
oxygen gas (the mass of 1 mole of oxygen) to produce 36.0 grams of H2O (the mass of 2 moles of water). This
would give the scientist the correct ratio of hydrogen atoms to oxygen atoms to produce water. (Remember
we need 2 H for every 1 O in the formula H2O).
Look at following steps to calculate the gram formula mass of the following compound:
Na2CO3
1. Count the number of each type of atom: 3. Multiply the number of atoms by the
a. 2 Na mass of each element and add all of the values:
b. 1 C a. Na: 2 x 23.0 = 46.0 g
c. 3 O b. C: 1 x 12.0 = 12.0 g
2. Look up in the Periodic Table the atomic mass c. O: 3 x 16.0 = 48.0 g
of each element. (Round to the nearest tenth) 46.0 + 12.0 + 48.0 = 106.0 g/mol
a. Na= 23.0 g
b. C= 12.0 g
c. O = 16.0 g
This is the amount of grams of Na2CO3 contained in 1 mole.
1 mole, or 6.02 x 1023 molecules, of sodium carbonate
(Na2CO3) weighs 106.0 grams and a scientist could indeed measure
106.0 grams using a lab balance.
