Question

$26.8$ gm of ${Na}_2{SO}_4.n{H}_{2}O$ contains $12.6$ gm water . The value of n is ?

• A. 1
• B. $10$
• C. 7
• D. $4$

Answer 1

Answer: (C)

7

To get the value of n; calculate the moles of water and that of sodium sulphate and then get the whole number ratio:

$Molesofwater:$

mass $H_{2}O=12.6$

molar mass= 18

moles=$\dfrac{mass}{molar mass}

=$\frac{12.6}{18}$

$=0.7$

$Molesofsodiumsulphate:$

mass$=26.8-12.6=14.2$

molar mass$=(23\times 2)+(32+64)=142$

moles $=14.2142$

$=0.1$

$\therefore$ ratio of moles of $N{a}_{2}S{O}_4:H_{2}O=0.1:0.7$

hence the whole number ratio = 1:7 (meaning there are 7 molecules of water in this hydrated salt)

Hence n=7