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ICSE Class 10 Chemistry 2014 Paper

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Question : 33 of 88
Marks: +1, -0
A compound made up of two elements XX and YY has an empirical formula X2X_2 YY. If the atomic weight of XX is 10 and that of YY is 5 and the compound has a vapour density 25 , find its molecular formula.
Solution:  
Emp. formula =X2Y=X_2 Y , At. wt. of X=X= 10, At. wt. of Y=5Y=5
∴\therefore Empirical formula mass
=2×10+5=25=2 \times 10+5=25
vapour density V.D. =25=25
 Mol. Mass     =   V.D. ×2=25×2\text{ Mol. Mass } \;\; = \; \text{ V.D. } \times 2 = 25 \times 2
  =50\;=50
n  =   Mol. Mass  Emp. formula mass n \; = \; \frac{ \text{ Mol. Mass } }{ \text{ Emp. formula mass } }
  =  5025=2\;=\; \frac{50}{25}=2
 Mol. formula     =   Emp. formula ×2\text{ Mol. formula } \;\; = \; \text{ Emp. formula } \times 2
  =X2Y×2\;= X_2 Y \times 2
  =X4Y2.\;= X_4 Y_2 .
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