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Date November 2017 Marks available 1 Reference code 17N.3.sl.TZ0.2
Level SL Paper 3 Time zone TZ0
Command term Calculate Question number 2 Adapted from N/A

Question

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]
a.

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]
b.

Calculate the uncertainty in the change in pH.

[1]
c.

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]
d.

Markscheme

Mg(OH)2 (s) + 2HCl (aq) → MgCl2 (aq) + 2H2O (l)

Accept full or net ionic equation.

a.

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]

b.

(±) 0.04

OR

(±) 0.03

c.

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]

d.

Examiners report

[N/A]
a.
[N/A]
b.
[N/A]
c.
[N/A]
d.

Syllabus sections

Core » Topic 11: Measurement and data processing » 11.1 Uncertainties and errors in measurement and results
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Core » Topic 11: Measurement and data processing
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