Question

What volume of hydrogen gas at 273 K and 1 atm pressure will be consumed to obtain 21.6 g of elemental boron (atomic mass = 10.8) from the reduction of boron trichloride by hydrogen?

• A. $67.2l$
• B. $44.8l$
• C. $22.4l$
• D. $89.6l$

Answer 1

The correct option is A $67.2l$

The underlying reaction is:
$2BC{l}_3+3H_2⟶2B+6HCl$
Mass of boron formed = 21.6 g,
Moles of boron: $\frac{mass}{molarmass}=\frac{21.6g}{10.8g/mol}=2mol$
Two moles of boron are formed, which means that 3 mol of hydrogen gas is consumed from the stiochiometry above.
Since 1 mol of gas at STP occupies 22.4 l, 3 moles would occupy:
$3\times 22.4=67.2l$