Question

How would the molar mass of the unknown be affected (increase, decrease, no change) if the following occurred during the lab? Explain.

a. Some unknown solids were spilled while transferring into the beaker.

b. The buret was rinsed with DI H2O but not conditioned with the 0.20 M NaOH solution.

Answer 1

a.

Some of the solids were spilled, so taken mass is less than the actual mass.

Molar Mass = Mass / Molarity.

Since, the mass is lower (taken mass). So, the molarity calculated with the experiment will give a lower Molar mass.

b.

If you are performing a titration with NaOH solution and you rinse the buret with DI H2O. When you rinse the buret, some of the liquid you rinsed it with remains in there. If you rinse it with DI H2O, the water left in the buret contains no NaOH and therefore the molarity of your NaOH solution is lightly lower than recorded. You would measure the acid to be slightly stronger than it actually is.

Now,

Molar Mass = Mass / Molarity

Since molarity of the acid will be higher. So, molar mass (recoded) will be lower.