pH and indicators
3 marks · NormalA student adds methyl orange to a solution. The solution turns yellow. They mix the solution with just enough acid to make the colour just change to red.
Estimate the change in pH that the student has achieved. State your reasoning.
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Show answer
Decrease of about 4 units (from pH ~11/above to pH ~3/below), since methyl orange is yellow in alkali (above pH 4) and red in acid (below pH 3).
Also accept: Decrease of 4 or more units; correct explanation with pH ranges
Common mistakes: Not relating to pH at which methyl orange changes colour; Using litmus or phenolphthalein ranges incorrectly
Show mark scheme and worked solution
Marking points
M1 · 1 mark · States the correct pH range for methyl orange colour change (yellow in alkali, red in acid; transition pH ~3.4–4.4).
M1 · 1 mark · Estimates pH before as above ~pH 4 (alkaline side), after as below ~pH 3 (acidic side).
A1 · 1 mark · Correctly calculates or states pH decrease (~4 units) and supports with explanation of colour change.
Worked solution
Methyl orange is yellow in solutions with pH above 4 and red below pH 3.
Adding just enough acid turns it from yellow to red, crossing from above pH 4 to below pH 3.
So, the pH changed by about 4 units (from above 7 or 11 to below 3).