PAG 11 – pH Measurement: Module 1: Practical Skills in Chemistry: Chemistry OCR A A Level

Give 3 ways you could identify whether a solution was acidic or basic.

1. Litmus paper

2. Indicator (e.g. universal indicator) and comparison to a colour scale

3. pH meter

How can litmus paper be used to identify an alkali?

Red litmus paper will turn blue.

Blue litmus paper will remain blue.

How can litmus paper be used to identify an acid?

Blue litmus paper will turn red.

Red litmus paper will remain red.

Why might a pH meter be used rather than universal indicator to measure pH?

1. Gives a numerical value for pH.

2. More accurate (typically read to 2 decimal places.

What is meant by the term ‘accuracy’?

How close the measurement is to the true value.

Briefly outline how to carry out a titration.

1. Use a pipette to measure the first solution into a conical flask.

2. Add a few drops of a suitable indicator.

3. Add the other solution into a burette and record the initial volume.

4. Slowly add the solution in the burette into the conical flask.

5. Swirl the mixture continuously until the end point is reached.

6. Repeat until concordant results are obtained.

What colour is methyl orange in:

a) Acid?

b) Alkali?

c) Neutral solution?

a) Red

b) Yellow

c) Orange

What is the colour of phenolphthalein in:

a) Acid?

b) Alkali?

a) Colourless

b) Pink

What is a Brønsted-Lowry acid?

A substance that donates protons (H+).

What is a Brønsted-Lowry base?

A substance that accepts protons (H+).

What is the difference between a strong and weak acid?

A strong acid completely dissociates in solution (readily releases H+ ions) whereas a weak acid only partially dissociates.

A solution has a pH of 1.00 when measured using a pH meter. Suggest the characteristics of this solution.

1. Acidic.

2. Likely to contain a strong acid.