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Chemistry - acids and buffers pH

This question is about vitamin C, C6H8O6.
Vitamin C is a weak monobasic acid with a Ka value of 6.76 × 10–5 mol dm–3.

Low acidity vitamin C tablets are less acidic than tablets containing just vitamin C.
A student dissolves a low acidity vitamin C tablet in water.
• The tablet contains a mixture of 300mg of vitamin C, C6H8O6, and the sodium salt of vitamin C, C6H7O6Na.
• The pH of the solution is 4.02.
(i) Calculate the ratio C6H7O6– : C6H8O6 in the solution.
Ans: 0.708:1

Calculate the mass of C6H7O6Na, in mg, in the low acidity vitamin C tablet.

I have no clue how to do this question...it’s only one mark so it must be simple I’m just not getting it.
The answers says it’s
0.708 x (300/176) x 198 = 239
Can someone explain how to get this??
Thank you
Reply 1
Avatar for Crow_M
Crow_M
22
Original post by Jfueofkrnb
This question is about vitamin C, C6H8O6.
Vitamin C is a weak monobasic acid with a Ka value of 6.76 × 10–5 mol dm–3.

Low acidity vitamin C tablets are less acidic than tablets containing just vitamin C.
A student dissolves a low acidity vitamin C tablet in water.
• The tablet contains a mixture of 300mg of vitamin C, C6H8O6, and the sodium salt of vitamin C, C6H7O6Na.
• The pH of the solution is 4.02.
(i) Calculate the ratio C6H7O6– : C6H8O6 in the solution.
Ans: 0.708:1

Calculate the mass of C6H7O6Na, in mg, in the low acidity vitamin C tablet.

I have no clue how to do this question...it’s only one mark so it must be simple I’m just not getting it.
The answers says it’s
0.708 x (300/176) x 198 = 239
Can someone explain how to get this??
Thank you


Hey, sorry that you didn't get an answer for this.

First part of the question:

KA value of vitamin C = 6.76x10^-5moldm-3
KA value of salt of vitamin C = reverse log (10^-4.02) = 9.55x10^-5moldm-3
Ratio of Vitamin C : Salt of Vitamin C = 6.76x10^-5moldm-3 : 9.55x10^-5moldm-3
To find the ratio you divide 6.76x10-5 by 9.55x10-5 which gives you 0.7078534031... rounding that, it gives you 0.708:1 which is the answer.

Second part:
(Ratio of vitamin c x moles of vitamin C) x Molar Mass (M) of salt of vitamin C.

So, to calculate the answer, you have the find M of vitamin C (C6H8O6) and (C6H7O6Na)
Carbon has a mass of 12, H a mass of 1.01,Oxygen a mass of 16 and Sodium a mass of 23.
Molar Mass (M) of C6H8O6 = (12x6) + (8x1.01) + (16x6) = 72+7.07+96= 176 rounded to 3 significant figures.
Molar Mass (M) of C6H7O6Na = (12x6) + (7x1.01) + (16x6) + (1x23) = 72+7.07+96+23= 198 rounded to 3 significant figures.

Getting the equation again... (0.708 x mass of the tablet (300) divided by the molar mass of vitamin C (176) times the molar mass of the salt (198)
That gives us 0.708 x (300/176) x 198 = 239.

Hope that this helps. :smile:
Are you in Year 12 by the way?
Reply 2
Avatar for Jfueofkrnb
Jfueofkrnb
OP
10
Thank you that makes a lot more sense
Year 13 actually:smile:

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