Question

Sea water is $3.5\%$ by mass of common salt and has a density $1.04\, g\, cm^{-3}$ at $293 \,K$. Assuming the salt to be sodium chloride, then osmotic pressure of sea water will be (assume complete ionisation of the salt)

• A. $25.45 \,atm$
• B. $11.56 \,atm$
• C. $29.98 \,atm$
• D. $30.20 \,atm$

Answer 1

Answer: (C)

Let mass of solution be $100\, g$.
Volume of solution $= \frac{Mass}{Density} = \frac{100}{1.04}$
$= 96.154 \,mL = 0.096 \,L$
$w_{B} = 3.5 \,g$
$M_{B} = 58.5\, g \,mol^{-1}$ (molar mass of $NaCl$)
For complete ionisation of $NaCl$, van’t Hoff factor will be $'2'$.
$i = 2$
we know,
$\pi = iCRT = i \frac{n}{V}RT$
$= 2 \times \frac{3.5 \times 0.0821 \times 293}{58.7 \times 0.096} = 29.98 \,atm$