Question

A mixture of $1.65\times {10}^{21}$ molecules of X and $1.85\times {10}^{21}$ molecules of Y weighs 0.688 g. If molecular mass of Y is 187, what is the molecular mass of X?

• A. $41.35$
• B. $42.35$
• C. $41.65$
• D. $41.75$

Answer 1

Answer: (C)

$41.65$

187g of Y$=6.022\times {10}^{23}$ molecules of Y

$1.85\times {10}^{21}$ molecules of Y$=\frac{187}{6.023\times {10}^{23}}\times 1.85\times {10}^{21}=0.574gm$ of Y

Total mass given$=0.688g$

Remaining$=(0.688-0.574)=0.114g$

So,$1.65\times {10}^{21}$ molecules of X will have 0.114 g mass.

So $6.022\times {10}^{23}$ will have$=\frac{0.114}{1.65\times {10}^{21}}\times 6.022\times {10}^{23}gm=41.6gm$