Question

Units of parts per million (ppm) or parts per billion (ppb) are often used to describe the concentrations of solutes in very dilute solutions. The units are defined as the number of grams of solute per million or per billion grams of solvent. Bay of Bengal has $1.89$ ppm of lithium ions. The molality of $L{i}^{+}$ in this 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 ions corresponds to $1.89$ g of lithium in $1000000$ g of solvent.

Thus, $1$ kg or $1000$ g of solvent will contain $1.89\times {10}^{-3}$ g of lithium.

This corresponds to $\frac{1.89\times {10}^{-3}}{7}=2.7\times {10}^{-4}$ mol of lithium.

$Molality\left(m\right)=\frac{Molesoflithium}{Massofsolvent\left(inkg\right)}=\frac{2.7\times {10}^{-4}}{1}=2.7\times {10}^{-4}m$