I. When reading the buret in the lab, it is read toof a mL. Notebook page (2 pts
ID: 546212 • Letter: I
Question
I. When reading the buret in the lab, it is read toof a mL. Notebook page (2 pts) relerence 2. A 0.500-gram sample of a weak, nonvolatile monoprotic acid, HA, was dissolved in sufficient water to make 50.0 mililiters 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. 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.(2 pts) b. Extra water is added to the 0.500-gram sample of HA. (2 pts) (a) An indicator that changes color at pH S is used to signal the equivalence point. (2 pts) b) An air bubble passes unnoticed through the tip of the buret during the titration.(2 pts)Explanation / Answer
2. Accuracy in the calculation of molar mass of an acid through an acid/base titration depends directly on the error in the experimental end point value, as that is used to find the concentration of the acid in 0.5g by the law of equivalence as VaMa = VbMb where the suffixes a and b stand for acid and base and Ma = (VbMb)Va. From the molarity of HA, we find that Ma moles of HA is in 0.5g and thus, the molar mass in terms of g/mol is calculated by MHA = 0.5/Ma.
Thus, if the equivalence point observed is arrives after the actual point, Vb will be more than required, giving higher Ma, which in turn will give a lower molar mass for HA. Similarly, if the equivalence point observed is before the actual, the molar mass calculated will be higher than actual.
a) If the standard solution is filled in right after rinsing the buret with distilled water, the small quantity of water present in the buret from rinsing will add to the standard, diluting it. Thus, the concentration of base will be lesser than that of the standard's used in calculation and lower concentration will also mean a higher volume of base required for neutralization. Hence, here the molar of HA will be lower than actual. NOTE: But since this dilution is not as extensive as elucidated and only greatly minimal, one can consider the change in molar mass to be in an agreeable margin of error too.
b) If extra water is added to the HA solution prepared, it will decrease the concentration of the acid pipetted out for titration, resulting in a premature equivalence point. Thus, this will result in a higher molar mass of HA than actual.
c) All acid/base titration involve neutralization reactions, resulting in a pH of 7 always at the equivalence point. Hence, in case of a base being the buret solution, if the indicator signals end point by colour change at pH = 5, lesser than required quantity of base to afford complete neutralization will be taken as the final volume of base required for neutralization as pH in the pipette solution increases from less than 7 in the beginning up to 7 at the equivalence point. Therefore, this will result in a higher calculated molar mass of HA.
d) If an air bubble passes through the buret tip, it will result in a sudden and considerable increase in the volume of buret solution drained to the observer while no such abnormal flow has actually occurred, resulting in a higher than actual volume of base at the equivalence point. So, the molar mass of the acid thus calculated will be lower than actual.