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VCE Stuff => VCE Science => VCE Mathematics/Science/Technology => VCE Subjects + Help => VCE Chemistry => Topic started by: Charmz on December 20, 2011, 01:44:22 pm

Title: Sig Fig question
Post by: Charmz on December 20, 2011, 01:44:22 pm

Why is the answer expressed in 3 sig figs when it's supposed to be in 4 (unless I'm mistaken)

Determine the percentage composition of the following compounds:
   lead(IV) oxide (PbO2)
         % Pb = 86.6220%
      Express the answer with the correct number of significant figures.
         % Pb = 86.6% (the suggested answer)


And where is the questions megathread thing?

Title: Re: Sig Fig question
Post by: Panicmode on December 20, 2011, 02:35:32 pm

Quote from: Charmz on December 20, 2011, 01:44:22 pm

Why is the answer expressed in 3 sig figs when it's supposed to be in 4 (unless I'm mistaken)

Determine the percentage composition of the following compounds:
   lead(IV) oxide (PbO2)
         % Pb = 86.6220%
      Express the answer with the correct number of significant figures.
         % Pb = 86.6% (the suggested answer)


And where is the questions megathread thing?

I'd need to be able to see the entire question (how many figures each measurement was given to) to give you a precise answer, but if you're working with the Heinemann book I can tell you that there are many mistakes (particularly where sig figures are concerned) in the answers.


As to the megathread, there was a vote a while back and it was decided that they would be removed to be replaced by "topic" threads. In the mean time feel free to keep asking questions on here :)

Title: Re: Sig Fig question
Post by: Charmz on December 20, 2011, 03:35:46 pm

That was the entire question.

Determine the percentage composition of the following compounds:
   lead(IV) oxide (PbO2)

Title: Re: Sig Fig question
Post by: Aurelian on December 20, 2011, 03:49:03 pm

You are correct Charmz.

M(PbO2) = 207.2 gmol-1 + 32.0gmol-1

When you're adding experimental data, you use the smallest number of decimal places.

As a result, M(PbO2) = 239.2 gmol-1, which is 4 significant figures.

I suspect that Heinmann's thinking concerns the oxygen in isolation. For example, if you found the % of oxygen, you would go (32.0/239.2)*100, and then if you subtracted that from 100% to find the % of Pb, your figure would be to three significant figures.

Title: Re: Sig Fig question
Post by: Charmz on December 20, 2011, 04:58:54 pm

Thank you :) I have found many other mistakes in the book so I'll know not to trust the book too much.