Question

A sample of pure calcium weighing $1.35$g was quantitatively converted to $1.88$ g of pure calcium oxide.

The atomic mass of calcium would be:

• A. $20$
• B. $40$
• C. $80$
• D. $10$

Answer 1

The correct option is B $40$

Let X g/mol be the atomic mass of calcium. The molecular mass of calcium oxide will be $X+16$ g/mol.

The number of moles of calcium $=\frac{1.35}{X}$

The number of moles of calcium oxide $=\frac{1.88}{X+16}$

$Ca+O\to CaO$

The number of moles of calcium $=$ number of moles of calcium oxide

$\frac{1.35}{X}=\frac{1.88}{X+16}$

$1.88X=1.35(X+16)$

$1.88X=1.35X+21.6$

$0.53X=21.6$

$X=40$

Therefore, the atomic mass of calcium would be $40$ g/mol.