general quem question please help!!!

[danbarr]

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[/B]okay I don't know why i can't figure this question out.......is in achiever test 2 question #59 general quem

what is the molality of 85% phosphoric acid H3PO4, if the density of the solution is 1.70 g/ml assuming we have 85 g of solute 15 g of solvent and 100g of total solution??? the answer achiever gives is

B. 1000*(1/1.70)*(100.0/15.0)*(1.70/1)*(85.0/100.0)*(1/98)m


what the hell??? i've been trying to read up on my review notes and textbook but i can't seem to find any question like this......my guess is a lot of convertions thrown in there.......can anybody please help me with this convertions and explain them to me........thank you guys as usual

danbarr

 

[slayerdeus]

So we have to get to (moles solute)/(kg solvent).

I donno what they did, but first I convert 15g to kg to get 0.015 kg of solvent. Then converted 85 grams to moles by dividing 85 by 98 (the MW of H3PO4). Then I just used the formula for molality (moles solute)/(kg solvent) and I got 57.8m which is the same answer as Achiever. I assume they tried to use the density but you don't have to.

 

[slayerdeus]

By the way in Achiever's answer the density cancels out as well as the 100. So essentially it is the same way I did except they tried to use every number which makes it confusing and tedious.

 

[danbarr]

thank you......I guess I need to take a coffee brake!!! i didn't even realize that the densities and the 100's cancell out.......one quick question though....what is the 1000 for....to convert to kg??? anyhow your help was desperately needed I was panicking here........thank you so so so much
danbarr

 

[slayerdeus]

thank you......I guess I need to take a coffee brake!!! i didn't even realize that the densities and the 100's cancell out.......one quick question though....what is the 1000 for....to convert to kg??? anyhow your help was desperately needed I was panicking here........thank you so so so much
danbarr

Yea 1000 is to convert to kg. However they don't really explain much here about why and where they put it. It should read (85/98)/(15/1000) and this gives you your answer, this is why 1000 is actually multiplied because you are dividing by the fraction. So mathematically what happens is (85*1000)/(98*15).