# questin 8.5 new aqa specimen paper

Discussion in 'Science' started by urvipp1982, Apr 14, 2018.

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1. ### urvipp1982New commenter

Dear All, i am trying to help a neighbours daughter with her chemistry and for the life of me i cannot figure out where they got the 0.0027 from. converting cm3 into dm3 is /1000 which equates to 0.027. so am confused. Please help.

The student carried out five titrations. Her results are shown in Table 5.

A student used a pipette to add 25.0 cm3 of sodium hydroxide of unknown concentration to a conical flask.
The student carried out a titration to find out the volume of 0.100 mol/dm3 sulfuric acid needed to neutralise the sodium hydroxide.

Titration 1 27.40

Titration 2 -28.15

Titration 3 27.05

Titration 4 27.15

Titration 5 27.15

Volume of 0.100 mol/dm3 sulfuric acid in cm3

4 Concordant results are within 0.10 cm3 of each other.

Use the student’s concordant results to work out the mean volume of 0.100 mol/dm3 sulfuric acid added.

[2 marks] 27.12

5 The equation for the reaction is:

2NaOH + H2SO4 → Na2SO4 + 2H2O

Calculate the concentration of the sodium hydroxide.
Give your answer to three significant figures.

[4 marks]

mark scheme says :
Moles H2SO4 = conc × vol =

0.00271 - but how did they get this figure. am i being a complete nunz? as 27.12 /1000= 0.02712, where has the extra 0 come from?

Ratio H2SO4:NaOH is 1:2 or

Moles NaOH = Moles H2SO4 × 2

= 0.00542

Concentration NaOH = mol/vol

= 0.00542/0.025 = 0.2168

0.217 (mol/dm3)

2. ### ah3069Occasional commenter

to calculate moles of sulphuric acid, you do the volume multiplied by the concentration

so volume in cm3 is 27.12

Volume in dm3 = 27.12 / 1000
= 0.02712

The to calculate moles, you multiply concentration and volume:

concentration is 0.1 mol/dm3 from the information

moles = 0.1 x 0.02712
moles = 0.002712

Then the rest of the working from there involves:

molar ratio 1:2

Moles of NaOH = 2 x 0.002712
= 0.005424

Concentration = moles / volume
= 0.005424 / 0.025
= 0.217 mol/dm3

It may be easier to use the grid method of doing this, it seems to work better for the pupils I teach!