Calculations with equations involving solutions
4 marks · NormalA student carries out a reaction between sodium hydroxide and hydrochloric acid:
| Experiment | Volume of NaOH (cm³) | Concentration of NaOH (mol/dm³) |
|---|---|---|
| 1 | 20 | 0.25 |
| 2 | 15 | 0.25 |
| 3 | 25 | 0.50 |
For experiment 2, calculate the number of moles of NaOH used.
Using the equation, state the number of moles of HCl needed to react with the NaOH in experiment 2 and explain your answer.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) 0.00375 mol
(b) 0.00375 mol; The ratio NaOH:HCl is 1:1, so the number of moles of HCl needed is the same.
Common mistakes: Using wrong row of data; Not using 1:1 molar ratio
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · Converts 15 cm³ to 0.015 dm³ or uses correct formula.
A1 (a) · 1 mark · Correct calculation: 0.00375 mol. (follow-through)
M2 (b) · 1 mark · Identifies 1:1 molar ratio in equation and links to answer. (follow-through)
A2 (b) · 1 mark · Correctly states 0.00375 mol of HCl needed, with clear explanation. (follow-through)
Worked solution
a) Experiment 2: 15 cm³ = 0.015 dm³; number of moles = 0.015 × 0.25 = 0.00375 mol.
b) The equation shows a 1:1 ratio. So, moles of HCl needed = moles of NaOH used = 0.00375 mol.