A chemist determines the concentration of an aqueous solution to be...

The number of moles of solute is calculated by multiplying the concentration by the volume.

$n = C \times V$

$n = 0.224 \text{ mol L}^{-1} \times 3.4 \text{ L} = 0.7616 \text{ mol}$

The concentration $0.224 \text{ mol L}^{-1}$ has three significant figures, and the volume $3.4 \text{ L}$ has two significant figures. In multiplication, the final result must be reported to the least number of significant figures present in the measured quantities, which is two.

Rounding $0.7616 \text{ mol}$ to two significant figures gives $0.76 \text{ mol}$.

Answer: $0.76 \text{ mol}$