Question

Units of parts per million (ppm) or parts per billion (ppm) are often used to describe the concentrations of solute in very dilute solutions. The units are defined as the number of grams of solute per million or billion grams of solvent. The Bay of Bengal has 1.89 ppm of lithium ions. The molality of $L{i}^{+}$ in water is: (atomic number of $Li=7$).

• A. $1.5\times {10}^{-4}m$
• B. $1.7\times {10}^{-4}m$
• C. $2.5\times {10}^{-4}m$
• D. $2.7\times {10}^{-4}m$

Answer 1

The correct option is D $2.7\times {10}^{-4}m$

1.89 ppm of Lithium corresponds to

$1.89Liin1000000\left({10}^6\right)$ of solvent.

Thus,$1$ or $1000$ of solvent contains

$1.89\times {10}^{-3}Li$

$\therefore molesofLithium=\frac{1.89\times {10}^{-3}}{7}=2.7\times {10}^{-4}$

Hence, molality(m) $=\frac{massofLithium}{massofsolventin}=2.7\times {10}^{-4}m.$