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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: whonut on May 08, 2014, 12:56:29 PM

Title: Question about the effect of error on subsequent calculated quantities
Post by: whonut on May 08, 2014, 12:56:29 PM

Hi

I have been doing a problem set where I have to figure out the number of water molecules in a hydrate. I first had to calculate the M_r of the hydrate from some titration data by the equation M_r=600W/mv and then find the number of water molecules, x by x=(M_r-149.3)/18.02. W is the mass of the hydrate that I used, 0.2g in this case. the question I'm struggling with is this:

'Calculate the effect of a 0.002g weighing error (in W) on a) your value of M_r and b) your value of x. Express your answers as %age uncertainties and explain why the uncertainty in x is greater than that in M_r'.

It looks to me, though, that p_x=p_Mr=p_W≈1%. The only way I could see them being different is if x was more dependent on W than M_r but I can't see how that's true from the equations. Dividing u_W by x and M_r would also obviously do it but that seems nonsensical to me.

Thanks in advance. Sorry if I don't make sense. I'll try to clarify anything that I need to.

Title: Re: Question about the effect of error on subsequent calculated quantities
Post by: mjc123 on May 09, 2014, 06:54:06 AM

Have you actually calculated the effect of the weighing error on M_r and x, as it asks? Write out your calculations in full and see if you can spot why the % uncertainty in x is greater than that in M_r. (By the way, what are mv and u_w?)