In a sample of pure calcium fluoride of mass 15.0 g, 7.70 g of calcium is present. First convert the mass into number of moles as follows:
Here, m is mass and M is molar mass.
Molar mass of Ca is 40 g/mol, putting the values,
Similarly, molar mass of is 78.07 g/mol thus, number of moles will be:
.
Thus, 0.1921 mol of have 0.1925 mol of Ca, or 1 mole of
will have approximately 1 mole of Ca.
Now, mass of Ca needs to be calculated in 45.0 g of . Converting mass into number of moles first,
Thus, number of moles of Ca will also be 0.5764 mol, converting number of moles into mass,
Therefore, mass of Ca will be 23.06 g.