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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: salam on September 15, 2016, 04:13:57 PM

Title: Right Number of Significant Digits
Post by: salam on September 15, 2016, 04:13:57 PM

I just wanted to check and see if my answer is to the right number of significant digits or not for the following question:

44.2 mL of 0.5000 M of AgNO3 is added to 270.0 mL of water which contains 5.832 g of K2CrO4. A precipitate of Ag2CrO4 forms. What is the concentration of the unprecipitated Cr2O4 2- (find the moles of the left over reagent and convert to concentration).

I got 0.06031 mol/L. Is that the right number of SD's (4) or should there be less?

Title: Re: Right Number of Significant Digits
Post by: Borek on September 15, 2016, 06:22:14 PM

Too many, for at least two reasons.

First - one of the number given has only 3 SD.

Second - rules for adding and subtracting numbers are a bit different. 0.1000+0.000001234 is not 0.100001234 but 0.1000.

Title: Re: Right Number of Significant Digits
Post by: docnet on September 16, 2016, 02:35:59 AM

sig figs are important! (my current chemistry teacher apparantly doesn't think so) ??? ::)

Title: Re: Right Number of Significant Digits
Post by: Borek on September 16, 2016, 03:00:11 AM

Quote from: docnet on September 16, 2016, 02:35:59 AM

sig figs are important!

They are not. They are a poor man's way of dealing with the propagation of error. Better than nothing, but they give you a false notion of having things under control. There are much better methods of treating errors in the calculations using uncertain data.

See https://en.wikipedia.org/wiki/Propagation_of_uncertainty