Question

a chemistry student must write down in his lab notebook the concentration of a solution of sodium hydroxide. the concentration of a solution equals the mass of whats dissolved divided by the total volume of the solution. heres how the student prepared the solution: the label on the graduated cylinder says: empty weight: 9.250 g he put some solid sodium hydroxide into the graduated cylinder and weighed it. with the sodium hydroxide added, the cylinder weighed 94.34 g. he added water to the graduated cylinder and dissolved the sodium hydroxide completely. then he read the total volume of the solution from the markings on the graduated cylinder. the total volume of the solution was 30.6 ml. what concentration should the student write down in his lab notebook? be sure your answer has the correct number of significant digits.

Explanation:

Step1: Calculate mass of sodium hydroxide

The mass of sodium hydroxide is the mass of the cylinder with sodium hydroxide minus the empty - weight of the cylinder. So, $m = 94.34\ g-9.250\ g = 85.09\ g$.

Step2: Calculate the concentration

Concentration $c=\frac{m}{V}$, where $m$ is the mass of the solute (sodium hydroxide) and $V$ is the volume of the solution. Given $V = 30.6\ mL$ and $m = 85.09\ g$. Then $c=\frac{85.09\ g}{30.6\ mL}\approx2.78\ g\cdot mL^{-1}$.

Answer:

$2.78\ g\cdot mL^{-1}$