Question

Ote

as
3
c
The concentration of lactic acid in yoghurt can be found by an acid-base
titration with sodium hydroxide.
In an experiment, 40-0 cm' of sodium hydroxide solution of concentration
0-100 mol dm exactly neutralised the lactic acid present in a 50-0 g tub of
yoghurt.
Giving your answers to three significant figures, calculate
I
the number of moles of sodium hydroxide used,
1:2 mole.
[1]
II
the number of moles of lactic acid that were present in the yoghurt,
assuming that I mole of sodium hydroxide reacts with 1 mole of lactic
acid,
1.2 mole.
[1]
III the mass of lactic acid (relative molecular mass 90-0) present in the
50-0 g tub of yoghurt,
Dx 3c
1.2 x 90.0
= 1089
IV the percentage, by mass, of lactic acid in the yoghurt.
100 =
46.3%
108
[1]
[1]
x10
3
Ans
1476797

Answer 1

I'm sorry, I believe there is a mistake in your calculation.

To calculate the number of moles of sodium hydroxide used, you need to use the formula: moles = (concentration x volume) where the concentration is given in mol/dm^3 and the volume is given in dm^3.

so in this case, number of moles = (0.100 x 0.040) = 0.004 moles.

II

To calculate the number of moles of lactic acid that were present in the yoghurt, we can use the stoichiometry of the reaction between sodium hydroxide and lactic acid. Since the reaction is a 1:1 reaction, the number of moles of lactic acid will be equal to the number of moles of sodium hydroxide used, which is 0.004 moles.

III To calculate the mass of lactic acid present in the yoghurt, we can use the formula mass = moles x molar mass where molar mass of lactic acid is 90.0 g/mol

so the mass of lactic acid = 0.004 x 90.0 = 0.36 g

IV To calculate the percentage of lactic acid by mass in the yoghurt, we can use the formula % by mass = (mass of lactic acid / mass of yoghurt) x 100

so the percentage of lactic acid by mass in the yoghurt = (0.36 / 50.0) x 100 = 0.72 x 100 = 72%