achiever GC question

[jjw822]

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It gen chem question 59 from test 2 .

question is what is molality of 85% H3PO4 where density is 1.7g/ml

answer is 1000 x (1/1.17) x (100/15) x (1.7/100) x (85/100) x (1/98)m

i get everything except the first two multiplication. achiever explained that those two multiplications are intended to express inverse of solvent weight in kg. I just dont get it.

I always did it without first two muliplication on destroyer questions..

can anyone please explain this to me???

THX!!

 

[UndergradGuy7]

It gen chem question 59 from test 2 .

question is what is molality of 85% H3PO4 where density is 1.7g/ml

answer is 1000 x (1/1.17) x (100/15) x (1.7/100) x (85/100) x (1/98)m

i get everything except the first two multiplication. achiever explained that those two multiplications are intended to express inverse of solvent weight in kg. I just dont get it.

I always did it without first two muliplication on destroyer questions..

can anyone please explain this to me???

THX!!

Is the answer 57.8m?

You can do 100 g solution * 0.85 = 85g of H3PO4 and if 85% is H3PO4 then 15% is H2O so 100 * 0.15 = 15 g H2O

Convert to moles of H3PO4 (85g H3PO4)(1 mole H3PO4/ 98g H3PO4)
Convert 15 g H2O to kilograms. 15 g of H2O * (1 kg/1000g)

If you put it all together (divide the moles of phosphoric acid by the kilogram equation) (85g H3PO4)(1 mole H3PO4/ 98g H3PO4)*(1/15)(1000/1) =57.8m

Or

If it asks What is the molality of 85% phosphoric acid, H3PO4, if the density of the solution is 1.70 g/ml?

density is 1.70 g of solution / ml of solution

so if the solution is 85% h3po4 then 0.85*1.70=1.445grams of phosphoric acid in 1 ml of solution
If the solution is 85% phosphoric acid then 100-85=15% of solvent.
So 1.70*0.15=0.255 grams of solvent in 1 ml of solution = 2.55 x 10^-4 kg of solvent

So 1.445 grams of phosphoric acid/(98 g/mol) = 0.014744898 moles of phosphoric acid

so 0.014744898 moles phosphoric acid/2.44 x 10^-4 kg solvent = 57.823 molality



Also I think you typed the answer from achiever wrong. It is supposed to be (1000)(1/1.70)(100/15)(1.70)(85/100)(1/98)m?