Question

A chemistry student must write down in her lab notebook the concentration of a solution of sodium hydroxide. The concentration of a solution equals the mass of what's dissolved divided by the total volume of the solution. Here's how the student prepared the solution: The label on the graduated cylinder says: empty weight: 1.5 grShe put some solid sodium hydroxide into the graduated cylinder and weighed it. With the sodium hydroxide added, the cylinder weighed 13.95 g.She added water to the graduated cylinder and dissolved the sodium hydroxide completely. Then she read the total volume of the solution from the markings on the graduated cylinder. The total volume of the solution was 127.53 ml. What concentration should the student write down in her lab notebook?

Answer 1

Answer:

The student should write the concentration as 9.72 x 10⁻² g/mL.

Explanation:

In this problem, the mass of sodium hydroxide is the difference between the initial mass of the cylinder and the mass of the cylinder after the addition of sodium hydroxide. It should be noted here that the figures should be rounded off according to the rules of significant figures. The mass is given as,

$m_{NaOH} = m_{final} - m_{initial}$  

$m_{NaOH} = 13.95 g- 1.5g\\ \\m_{NaOH} = 12.4 g$

Here, only one figure is kept after the decimal. Among the two numbers whose difference is calculated, the number 1.5 contains one significant figure after its decimal. So, the answer is rounded off to one significant figure after the decimal.

The volume of the solution in the cylinder was 127.53 mL. So, the concentration of NaOH will be:

$C_{NaOH}= \frac{m_{NaOH}}{V(mL)}$

$C_{NaOH}= \frac{12.4g}{127.53mL}$

$C_{NaOH}= 9.72\times 10^{-2} g/mL$

According to the rules of significant figures for division, the answer should be rounded off to the least number of significant figures in the problem. Here, it is three significant figure due to the number 12.4 in the numerator.

Answer 2

Answer:

97.624 g/l.

Explanation:

Mass of NaOH + empty cylinder = mass of dry NaOH + mass of empty cylinder

= 13.95 - 1.5

= 12.45 g.

Total volume of solution = 127.53 ml.

In l, 0.12753 l of solution.

Concentration of the NaOH solution = mass of dry NaOH/total volume of solution

= 12.45/0.12753

= 97.624 g/l.