A 0.500-gram sample of a weak, nonvolatile monoprotic acid. HA. was dissolved in
ID: 1048603 • Letter: A
Question
A 0.500-gram sample of a weak, nonvolatile monoprotic acid. HA. was dissolved in water to make 50.0 milliliters of solution. The solution was then titrated with a stand 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. 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. Extra water is added to the 0.500-gram sample of HA. An indicator that changes color at pH 5 is used to signal the equivalence point. An air bubble passes unnoticed through the tip of the buret during the titration.Explanation / Answer
a. Not affected: Molar mass of acid can be calculated by measuring the volume and hence moles of NaOH conusmed for neutralization of acid by titrating it to its equivalence point.
b. Not affected: The amount of acid present in the solution remains same even after addition of extra water.
(a) Too high: A detector which changes colour at pH 5 can not be used to signal end point because at the equivalence point of weak acid and strong base the resulting solution is slightly alkaline. Therefore, to estimate the volume of NaOH used for neutralization of the acid, indicator such as phenolphthalein whose colour changes between the pH range of 8-10 to be used.
(b) Too low: The presence of air bubble will read the volume of NaOH in buret slightly more than its actual value.If the bubble get passed from the buret, an additional amount of volume will be lost from buret reading which in reality is a false reading and will cause error in the result.
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