Question

2. A 0.500-gram sample of a weak, nonvolatile monoprotic acid, HA, was dissolved in sufficient water to make 50.0 milliliters of solution. The solution was then titrated with a standard NaOH solution. Predict how the calculated molar mass of HA would be affected (too high, too low, or not affected) by the following laboratory procedures. Explain each of your answers. A. (2 points.). After rinsing the buret with distilled water, the buret is filled with the standard NaOH solution; the weak acid HA is titrated to its equivalence point. B. (2 points.) Extra water is added to the 0.500-gram sample of HA. C. (2 points.) An indicator that changes color at pH 5 is used to signal the equivalence point. D. (2 points) An air bubble passes unnoticed through the tip of the buret during the titration.

Answer 1

A. After rinsing with distilled water , there will be certain amount of water in the burette. Hence , it will decrease the strength of the basic solution in the burette. We calculate the strength by using the formula V1S1 = V2S2 where , V1 = volume of weak acid taken in the conical , S1 = strength of the weak acid , V2= volume of the base added , S2 = strength of the base added in the system.

So , when the strength of the base is decreased, more amount of the basic solution will have to be added . So , in calculation , when the volume is written more than the usual , the calculated strength of the base will be lower than usual. Hence , alternatively the calculated molar mass of the weak acid will be high.

B. Adding extra water means diluting the solution . So , diluting the solution will decrease the volume of base to be added . Hence , molar mass of the acid will be lower than usual.

C. Normally the equivalence pH of the titration of weak acid and strong base will be around 8. Hence , using an indicator of pH 5 will show colour change at around pH 5 , which is actually lower value than equivalence point. Hence , the calculated volume of base will be low which eventually get lower the molar mass value of the weak acid.

D. When a bubble is passed in the burette , it will show a higher value than added amount of base. Hence , the added value of base will be higher than usual ,which will increase the molar mass will be higher than ususal.