Self-assessment
Question 1 of 6
Which of the following statements are true?
A
The ionic product of water can be used to calculate the concentration of either H3O+ or OH– in solution if the concentration of the other ion is known.
B
Kw is known as the ionic product of water and has a value of 10–14 at 25 °C.
C
The concentration of H3O+ in pure water is always 10-7, and therefore pure water is always pH7.
D
In pure water, for every H3O+ ion produced, one OH– ion is also produced.
The hydroxide ion concentration of a solution is 2 x 10-11 mol dm-3. Using the ionic product of water, work out the hydrogen ion concentration.
A
–5 x 104 mol dm–3
B
2 x 10–4 mol dm–3
C
2 x 104 mol dm–3
D
5 x 10–4 mol dm–3
A student wishes to find out the concentration of H+ ions and OH– ions present in her solution. She measures the pH as 11.0 at 25 °C. Calculate the H+ ions and OH– ion concentrations.
A
1 x 10–10 mol dm–3 and 1 x 10–4 mol dm–3
B
1 x 10–11 mol dm–3 and 1 x 10–3 mol dm–3
C
1 x 10–11 mol dm–3 and 1 x 10–2 mol dm–3
D
1 x 10–10 mol dm–3 and 1 x 10–3 mol dm–3
What might you, the teacher, say to a student who has problems when it comes to putting numbers into Kw and pH calculations and who regularly forgets their own calculator?
A
Remind them how important it is to bring their calculator and know exactly which buttons to press for which calculations, eg EXP button, log button, inverse log.
B
Make sure that they know which buttons to press on each and every different model of calculator in class that they might borrow.
C
Ask the student to go home to collect their calculator.
D
Give the student more and more examples to do until they get the numbers right.
Manipulating whole powers of ten is simple without a calculator. Remembering a couple of other logs can help to mentally estimate the answer to trickier pH calculations such as pH 5.3 or 1.7. Which of the following logs would help most with these calculations?
A
log 5 = 0.7
B
log 2 = 0.3
C
log 2.5 = 0.4
D
log 4.5 = 0.65
Secure manipulation of exponentials, eg 1 x 10–10 is essential to pH and Kw calculations. What might be the problem if your student is consistently wrong by a factor of 10 in their answers?
A
insufficient practice with these calculations
B
typing 10 then the EXP button
C
getting the decimal point in the wrong place
D
the calculator needs new batteries.