Chemistry Tutoring

CaCO3 calcium carbonate, molecular mass explained

Note on units, amu is atomic mass units.

This unit is based on one twelfth the mass of carbon 12 = so carbon 12 mass = 12 amu.

Calcium (Ca) has atomic mass = 40.078 amu
Carbon (C) has atomic mass = 12.00 amu
Oxygen (O) has atomic mass = 15.999 amu

So adding these up in the ratio indicated by the molecular formula CaCO3
Ca x 1 = 40.078
C x 1 = 12.00
O x 3 = 47.997

==> 100.075

This means one molecule of CaCO3 weighs 100.075 amu.

Or one mole of CaCO3 weighs
100.075 g.

% of Ca in CaCO3 by mass
= 40.078 / 100.075 x 100 (this gives %)
= 40.0479 %

In 10g of CaCO3, there will be 4.05g of carbon,

Because,

10g of CaCO3
Means of the 10g, 40.0479 % is carbon. (From above math),

40.0479/100 x 10g = 4.00479 g