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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 3Cu + 3H2O + N2
(Volume occupied by 1 mole of gas at STP is 22.4 litres).

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Solution

(a)
Element Atomic mass Percentage Relative number of atoms Simplest ratio of moles
K 39 47.9 47.939=1.2 1.20.6=2
Be 9 5.5 5.59=0.6 0.60.6=1
F 19 46.6 46.619=2.4 2.40.6 =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 = 12080=1.5
Number of moles of ammonia required = 23×1.5 =1
Volume occupied by one mole of ammonia at STP is 22.4 L.

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