Question 17N.3.sl.TZ0.2
| Date | November 2017 | Marks available | [Maximum mark: 6] | Reference code | 17N.3.sl.TZ0.2 |
| Level | sl | Paper | 3 | Time zone | TZ0 |
| Command term | Calculate, Discuss, State, Suggest | Question number | 2 | Adapted from | N/A |
Antacids react with hydrochloric acid in the stomach to relieve indigestion. A student investigated different brands of antacid to see which caused the largest increase in pH in a given time. She added the antacids to hydrochloric acid, and recorded the change in pH over five minutes.
State an equation for the reaction of magnesium hydroxide with hydrochloric acid.
[1]
Mg(OH)2 (s) + 2HCl (aq) → MgCl2 (aq) + 2H2O (l)
Accept full or net ionic equation.
Suggest two variables, besides the time of reaction, which the student should have controlled in the experiment to ensure a fair comparison of the antacids.
[2]
Any two from:
volume «of HCl»
concentration «of HCl»/[HCl]
temperature «of HCl»
mass of antacid/tablets
size of antacid particles/tablets
OR
surface area of antacid «particles»/tablets
Accept “number of tablets/different doses”.
Do not accept “same pH meter” OR “initial pH” OR “concentration of antacid/[antacid]”.
A variable must be given so do not accept answers such as “stirring”, “whether tablets are whole or crushed” etc.
[Max 2 Marks]
Calculate the uncertainty in the change in pH.
[1]
(±) 0.04
OR
(±) 0.03
The student concluded that antacid B was the most effective, followed by A then C and finally D. Discuss two arguments that reduce the validity of the conclusion.
[2]
Any two of:
uncertainty «(±)0.04/(±)0.03» means A and C cannot be distinguished
each measurement was conducted once
stomach pH should not be raised a lot «so antacid B is not necessarily effective»
mass/number of tablets/dose «of antacid» used was not controlled
actual environment in stomach is different
Accept “amount of tablets” for “dose”.
Do not accept “nature/composition of tablets differs”.
Accept an answer such as “time frame is too short since some antacids could be long-acting drugs if they contain a gelatinisation/delaying agent” but not just “time frame is too short since some antacids could be long-acting drugs”.
[Max 2 Marks]