Question

Salt Hydrolysis Lab

Suggest how various experimental errors would affect your calculated values of Pka, Ka, and or molarity. Clearly explain whether the calculated value would be higher, lower, or no different than the actual value. Possible experimental errors may include:

a) The pH buffers used to calibrate the PH meters were 4.5 and 7.5, whereas you thought they were 4.0 and 7.0 and calibrated the pH meters accordingly.

b) You decided to take the pH after each addition of 5.0 mL: of NaOH rather than after every 0.5 mL during the neutralization of your acid sample.

c) While using the Half Volume method, your solution was dark pink (you overshot the endpoint) and you added the unreacted acid to this solution.

d) For the half volume method, you refilled the buret with 0.20 M NaOH rather than 0.1 M NaOH (Which is the value recorded in your notebook).

Answer 1

a) Molarity:

molarity is moles of solute per litre of solution. Now if actual pH is higher than the assumed pH, it means less acidic strength actuallly, which further implies less concentration of h+ ions in the solution. And as the concentration of ions per litre solution decreases, molarity also decreases.

Ka:

Ka is the equilibrium constant for the dissosiation of acid. More value of Ka indicates more shift of equilibrium towards product side, more dissosiation of acid, i.e. more H+ ions. In short, more acidic strength( ability of acids to give H+ ions easily) means more value of ka. so if actually pH is higher than the assumed one, means less acidic strength and hence less Ka.

pKa:

we know that , and so as Ka decreases pKa increases( because of the negative sign).

So here, as original pH is higher, we have lower Ka and hence higher value of pKa.

b) We calculate pH after every 0.5 ml addition of NaOH to eliminate the possibility of crossing the equivalence point and to maintain the accuracy of the experiment. If in between the addition of 5 ml of NaOH(say at 4 ml), the acid is neutralised, then that extra 1 ml will take solution towards basic nature. And the pH will increase.( more basic=more pH) and then the results will be as mentioned in the previous part of the question.

c) If the end point is overshot, it means the solution isn't neutral anymore but it has turned basic.So, if unreacted acid is added in the solution, pH will lower down and if the addition is accurate enough, it may turn neutral. Anyway, addition of acid dereases the pH, which means molarity is increased, ka is increased and pKa is decreased. ( reasons are same as explained in part 'a').

d) The equivalence point in half volume method is given by:

stoichiometric amount of titrant= titrated substance.

so, if the molarity of base(titrant) is increased, then as per the relation the volume of the base required decreases to maintain the relation. But that just shifts the end point a little earlier. i.e.now less amount of base will be required to neutralise the acid(titrand) taken. It doesn't changes the pH of the solution. so, there will be no change in molarity, Ka and pKa of the solution.