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Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: nj_bartel on January 25, 2009, 07:14:22 PM

Title: Error calculation going wrong (1 order of magnitude every time)
Post by: nj_bartel on January 25, 2009, 07:14:22 PM

Problem: The method described in Problem 5.6 is to be used for the analysis of ores that assay about 1.2% gold.  What minimum sample weight should be taken if the relative error resulting from a 0.4 mg loss is not to exceed -0.2%?

The referenced problem is: A method of analysis yields weights for gold that are low by 0.4 mg.  Calculate the percent relative error caused by this uncertainty if the weight of the gold in the sample is 900 mg.


My work: 

1.2% gold / 100% = 0.012 mg Gold / mg Ore

[x - (x + 0.04)] / (x + 0.04) * 100% = -0.2%

[x - (x + 0.04)] / (x + 0.04) = -0.002

-0.04 / (x + 0.04) = -0.002

-0.04 = -0.002x - 8*10^-5

-0.03992 = -0.002x

x = 19.96 mg Gold

(19.96 mg Gold) / (0.012 mg Gold / mg Ore) = 1660 mg Ore required for sample

The correct answer is 16600 mg Ore required for sample; where's my error?

Thanks a bunch

Title: Re: Error calculation going wrong (1 order of magnitude every time)
Post by: macman104 on January 25, 2009, 07:22:41 PM

I didn't check everything, but the first thing that jumped out was that you have 0.04, and they say 0.4 mg...

Title: Re: Error calculation going wrong (1 order of magnitude every time)
Post by: nj_bartel on January 25, 2009, 07:24:16 PM

That would do it... Thanks, lol