OCR A level Chemistry number of sig figs? Watch

john_iqbal786
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This question asks to find the number of molecules of an acid to an appropriate number of sig figs.
The data they give you is the mass is 38.25g and molar mass is 150.

The calcaulation is (38.25/150) * 6.02x10^23.

Since the mass is given to 4 sig figs and the other two are exact values, I put my answer in 4 sig figs. But the MS says it has to be in 3 sig figs due to one value being at 3 sig figs? Plz help
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pixiefuller
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(Original post by john_iqbal786)
This question asks to find the number of molecules of an acid to an appropriate number of sig figs.
The data they give you is the mass is 38.25g and molar mass is 150.

The calcaulation is (38.25/150) * 6.02x10^23.

Since the mass is given to 4 sig figs and the other two are exact values, I put my answer in 4 sig figs. But the MS says it has to be in 3 sig figs due to one value being at 3 sig figs? Plz help
Rounding to 4 sig figs makes sense, personally I always give my answers to 3 sig figs as that's what I got used to doing in Maths, and it has seemed to work out for me.
The mark scheme for our actual exam might be kinder, but I don't really know either. I'd just stick to giving everything to 3 sig figs just to be safe, unless otherwise told to by the question.
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john_iqbal786
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(Original post by pixiefuller)
Rounding to 4 sig figs makes sense, personally I always give my answers to 3 sig figs as that's what I got used to doing in Maths, and it has seemed to work out for me.
The mark scheme for our actual exam might be kinder, but I don't really know either. I'd just stick to giving everything to 3 sig figs just to be safe, unless otherwise told to by the question.
Thanks for the reply,

I've realised that the value used for avogadros constant is rounded to 3sf and therefore answer is expected in 3sf. Also, I know in maths 3sf is fine but in science they expect you to put the answer in the sig figs that the question is asked in. So if all values are given in 3sf but one value is in 2sf, they would want you to put it in 2sf. I've seen many answers where you would lose a mark for not doing this so you might want to look double check your answer with the mark scheme just to check if you have got the number of sf right
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Dysf(x)al
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(Original post by john_iqbal786)
This question asks to find the number of molecules of an acid to an appropriate number of sig figs.
The data they give you is the mass is 38.25g and molar mass is 150.

The calcaulation is (38.25/150) * 6.02x10^23.

Since the mass is given to 4 sig figs and the other two are exact values, I put my answer in 4 sig figs. But the MS says it has to be in 3 sig figs due to one value being at 3 sig figs? Plz help
You are supposed to use the fewest number of significant figures out of the values you use. 150 is to 3 sig figs (this is very iffy and I hate it, but I think it's safe to assume molar masses are to the nearest 1gmol-1 even if they end in 0), as is Avogadro's constant in the formula book.
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