21.4 Acid base titrations

Question 1

How does titration find the concentration of a solution

Answer 1

. You gradually add an acid of known concentration from a burette to a measured amount of solution in a base

. Do this until the indicator shows it has been neutralised

Or you can do it the other way by adding base to acid until acid is neutralised.

You can find the concentration of alkali from the volume of acid used

Question 2

What is another way to measure PH

Answer 2

. Use a PH meter which is calibrated by placing the probe in a buffer solution of known PH.

Question 3

What is a monoprotic acid

Answer 3

. Only one acidic hydrogen (one that can be donated to a base) .
HCl is monoprotic whereas H2SO4 is diprotic

Question 4

Describe the titration curve for when a strong base is added to a strong acid

Answer 4

. On the Y axis is PH scale, and it will start off low down on PH scale at around one or 2 and will end at around 13

. So the overall PH on Y scale changes a lot so there is a long vertical line at 25cm^3 of base added

. It is a weak gradient at the beginning and at the end

Question 5

PH graph for a strong acid and weak base

Answer 5

. On Y axis it will start off very low down on PH scale and will end at around PH 9
. Medium length vertical line at 25cm^3 of base added before it plateaus

Question 6

PH graph for a weak acid and weak base

Answer 6

. It will start on y axis at around PH 3 as its not strong acid, and there is an extremely short vertical line at 25cm^3
. The line plateaus at around PH 9 or 10

Question 7

PH graph for a weak acid and strong base

Answer 7

. Starts off on Y axis at around PH3 and ends at PH 13

. So medium length vertical line at 25cm^3

Question 8

What factors are in common with these PH graphs

Answer 8

. PH doesn’t change in a linear manner as the base is added.

Each curve has almost horizontal sections where lots of base is added without changing PH much

There is also a very steep part except with weak acid weak base, where a single drop can change PH a lot

Question 9

What is equivalence point of a graph

Answer 9

. Point where sufficient base has been added to neutralise the acid or vice versa.

. In each of the titrations in the examples above, the equivalence point is reached after 25cm^3 of base has been added

PH at equivalence point isn’t always 7

Question 10

How does curve look when acid is added to base

Answer 10

. The line will start high up on PH scale at around 13 as it is a strong base
. It will eventually plateau at around PH 1

Question 11

Describe how you would solve

. In titration the equivalence point is reached when 25cm^3 of 0.0150moldm-3 sodium hydroxide is neutralised by 15cm^3 of HCl.

What is the concentration of the acid

HCl + NaOH —> NaCl + H2O

Answer 11

. So equivalence point shows that 15cm^3 of HCl of concentration A has the same moles as in 25cm^3 of NaOH.

So find moles of NaOH and do ratio to get moles of HCl and then find concentration

So concentration of acid is 0.0250 moldm-3

Question 12

With a diprotic acid, describe how you would solve

. In titration, equivalence point is reached when 20cm^-3 of 0.0100moldm-3 of NaOH is neutralised by 15cm^3 sulfuric acid.

What is the concentration of the acid?

H2SO4 + 2NaOH –> Na2SO4 + 2H2O

Answer 12

. Equivalence point shows that 15cm3 of sulfuric acid of solution B has same number of moles as 20cm-3 of 0.01moldm-3 NaOH

So moles is 0.0002 of NaOH, so divide this by 2 to get moles of H2SO4, so 0.0001.
0.0001 / 0.015

So concentration is 6.67 X 10^-3