Concept of Atomic weight

Question:

!!P!! and !!Q!! are two elements which forms !!P_2Q_3!! and !!PQ_2!!. If !!0.15!! mole of !!P_2Q_3!! weights !!15.9g!! and !!0.15!! mole of !!PQ_2!! weights !!9.3g!!.

The atomic weight of !!P!! and !!Q!! is (respectively)

a) !!18, 26!!

b) !!26, 18!!

c) !!13, 9!!

d) None of above

Solution

!!2P +3Q -> P_2Q_3!!

!!P+ 2Q -> PQ_2!!

!!M_(P_2Q_3) = 15.9/0.15 = 2P+3Q...(1)!!

and

!!M_(PQ_2) = 9.3/0.15 = P+2Q...(2)!!

From equation (1) and (2), we get

!!=> P= 26 and Q= 18!!

Therefore, option (b) is correct.