William C. answered • 09/19/23
Experienced Tutor Specializing in Chemistry, Math, and Physics
a) Calculate the formula mass (molar mass) of the following compounds: Ca3(PO3)2, (NH4)2CO3, C2H5OH.
you calculate molar masses by adding up atomic mass numbers multiplied by the number of times the element occurs in the formula. For example
In the formula Ca3(PO3)2 Ca occurs 3 times, P occurs 2 times, and O occurs 6 times
The atomic masses of the atoms are 40.078 for Ca, 30.974 for P, and 15.999 for O
so the molar is 3(40.078) + 2(30.974) + 6(15.999) = 278.176
(NH4)2CO3 and C2H5OH molar masses are calculated by the same procedure.
b) Find the percentage by mass of each element in: C2H5OH, (NH4)2SO4, Ca3(PO3)2.
For each element take its atomic mass number multiplied by
the number of times the element occurs in the formula,
divide this by the molar mass,
then multiply the result by 100 to convert it to a percent
Ca3(PO3)2
percentage Ca = 100 × 3(40.078)/278.176 = 100 × 0.432 = 43.2 %
percentage P = 100 × 2(30.974)/278.176 = 100 × 0.223 = 22.3 %
percentage O = 100 × 6(15.999)/278.176 = 100 × 0.345 = 34.5 %
It's a good idea to check that your percentages add up to 100 % as they do here:
43.2 + 22.3 + 34.5 = 100.0 %
Percentage by mass of each element for (NH4)2CO3 and C2H5OH are calculated by the same procedure.
