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# Don't understand hwo the answer on the mark scheme is correct. watch

1. This is about an as-level chem past exam paper (q 19, 6CH02/01 june 2010)

The question refers to this part of a table;

Titration number . . . 2

Burette final . . . 28.00

Burette start . . . 0.05

volume used . . . 27.95

q) the burette used in the titration had an uncertainty for each reading of + or - 0.05cm^3.

What should be regarded as the true value of Tn 2?

x) 27.90 and 28.00
y) 27.925 and 27.975
z) 27.85 and 28.05

I put down x, but the mark scheme said z. is the curcial word EACH reading of the burette. both readings could have been 0.05 lower and higher which would be why the mark scheme says allow 27.95 +/- 0.1 cm^3

Is that why, it's the only thing i can think of . . .
2. (Original post by Nix-j-c)
This is about an as-level chem past exam paper (q 19, 6CH02/01 june 2010)

The question refers to this part of a table;

Titration number . . . 2

Burette final . . . 28.00

Burette start . . . 0.05

volume used . . . 27.95

q) the burette used in the titration had an uncertainty for each reading of + or - 0.05cm^3.

What should be regarded as the true value of Tn 2?

x) 27.90 and 28.00
y) 27.925 and 27.975
z) 27.85 and 28.05

I put down x, but the mark scheme said z. is the curcial word EACH reading of the burette. both readings could have been 0.05 lower and higher which would be why the mark scheme says allow 27.95 +/- 0.1 cm^3

Is that why, it's the only thing i can think of . . .
I pressume so
3. (Original post by Nix-j-c)
This is about an as-level chem past exam paper (q 19, 6CH02/01 june 2010)

The question refers to this part of a table;

Titration number . . . 2

Burette final . . . 28.00

Burette start . . . 0.05

volume used . . . 27.95

q) the burette used in the titration had an uncertainty for each reading of + or - 0.05cm^3.

What should be regarded as the true value of Tn 2?

x) 27.90 and 28.00
y) 27.925 and 27.975
z) 27.85 and 28.05

I put down x, but the mark scheme said z. is the curcial word EACH reading of the burette. both readings could have been 0.05 lower and higher which would be why the mark scheme says allow 27.95 +/- 0.1 cm^3

Is that why, it's the only thing i can think of . . .
Yes, you have to apply the error to each reading. So the start value could have been 0.05 - 0.05 = 0.00 and the final value could have been 28.00 + 0.05 = 28.05 (that gives you your max Tn 2). Similarly, the start value could have been 0.05 + 0.05 = 0.1 and the final could have been 28.00 - 0.05 = 27.95 (that gives you your min Tn 2)
4. Its because on each burette measurement you have 2 readings, the initial and the final and each of those has error +/- 0.05 so the error needs to doubled to 0.1 either side of the final value and thats why its z.

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