Question

A chemistry student must write down in her lab notebook the concentration of a solution of sodium thiosulfate. 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:2.2g

She put some solid sodium thiosulfate into the graduated cylinder and weighed it. With the sodium thiosulfate added, the cylinder weighed 19.440g.

She added water to the graduated cylinder and dissolved the sodium thiosulfate completely. Then she read the total volume of the solution from the markings on the graduated cylinder. The total volume of the solution was 50.29 mL.

What concentration should the student write down in her lab notebook? Be sure your answer has the correct number of significant digits

Answer 1

The atomic masses are

Na: 22.989 u

S: 32.065 u

O: 15.999 u

The gram molar mass of sodium thiosulfate, Na₂S₂O₃ = (2*22.989 + 2*32.065 + 3*15.999) g/mol = 158.105 g/mol.

Mass of Na₂S₂O₃ taken = (19.440 – 2.2) g = 17.240 g.

Mole(s) of Na₂S₂O₃ taken = (mass taken)/(molar mass)

= (17.240 g)/(158.105 g/mol)

= 0.1090 mole.

Volume of the solution = 50.29 mL = (50.29 mL)*(1 L)/(1000 mL)

= 0.05029 L.

Molar concentration of the sodium thiosulfate solution prepared

= (moles of sodium thiosulfate)/(volume of solution in L)

= (0.1090 mole)/(0.05029 L)

= 2.1674 mol/L

≈ 2.2 mol/L = 2.2 M

(ans, correct to 2 sig. figs; the least precise measurement has two sig. figs and the answer cannot be more precise than the least precise measurement).