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Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: BlueSourBalls on September 24, 2018, 08:02:51 PM

Title: Help with uncertainty
Post by: BlueSourBalls on September 24, 2018, 08:02:51 PM
Here is the following question: 

Pb(NO3)2 (aq) + 2 KI (aq)   →   2 KNO3 (aq) + PbI2(s)

If 20.50 (±0.05) mL of a 0.00105 M (±0.00002 M) KI stock solution that was diluted by 100, was required to create the yellow precipitate, what is the mass of the lead(II) nitrate in the sample?

I have gone up to the step on finding the mass of the lead(II) nitrate, however what I do not understand is that the moles that I have calculated for lead(II) nitrate has an uncertainty, but the molar mass that I have to look up does not have any uncertainty. Could someone explain what I should do in this case. I have attached 2 screenshots of what I have done with Part2.png following after Part1.png. Thanks!
Title: Re: Help with uncertainty
Post by: Borek on September 25, 2018, 02:49:10 AM
Technically molar mass does have uncertainty as well, it is just many orders of magnitude lower and as such doesn't show in the final result.