Question

(a) Determine the empirical formula of a compound containing 47.9% potassium, 5.5% beryllium and 46.6% fluorine by mass (Atomic weight of Be = 9; F = 19; K = 39). Work to one decimal place.
(b) Given that the relative molecular mass of copper oxide is 80, what volume of ammonia (measured at STP) is required to completely reduce 120 g of copper oxide? The equation for the reaction is:
3CuO + 2NH₃ $\stackrel{}{\to }$ 3Cu + 3H₂O + N₂
(Volume occupied by 1 mole of gas at STP is 22.4 litres).

Answer 1

(a)

Element	Atomic mass	Percentage	Relative number of atoms	Simplest ratio of moles
K	39	47.9	$\frac{47.9}{39}=1.2$	$\frac{1.2}{0.6}=2$
Be	9	5.5	$\frac{5.5}{9}=0.6$	$\frac{0.6}{0.6}=1$
F	19	46.6	$\frac{46.6}{19}=2.4$	$\frac{2.4}{0.6}=4$

Empirical formula of the compound is K₂BeF₄.

(b) According to the balanced chemical equation, for reduction of three moles of copper oxide, two moles of ammonia is required.
Number of moles of copper oxide in 120 g = $\frac{120}{80}=1.5$
Number of moles of ammonia required = $\frac{2}{3}\times 1.5=1$
Volume occupied by one mole of ammonia at STP is 22.4 L.