We did a lab with Copper Chloride (II) where we dehydrated a sample for 30 minutes, weighed it, heated it up for another 10 minutes, and then put it over boiling water and rehydrated the sample, Here's out numbers: First weigh (before dehydration):.589 g Second weigh (30 min in the oven): 17.501 g third (after an extra 10 min in the oven): 17.499 and after re-hyradtion: 1.256 g I need help finding the ratio of water to copper chloride in the sample.

and it's the mole ratio I need help with

Is the after rehydration weight 1.256kg

yeah that was are final number

My partner got back to me saying it was a 243.9 : 1 ratio, but no idea how he would get that number

k ...u wrote 1.256g in your question :P

you can safely assume that the weight of given amount of CuCl2 (completely dehydrated) is around 17.500g

yes

you can see this because after putting the sample in the oven for additional time - its weight does not reduce right??

Increase in weight after rehydration is -> (1256 - 17.5)g So this is the weight of the water that combines with the CuCl2

it reduced very slightly...taking the excess water that was left out

yeah it reduced very slightly because all the water was (almost) already taken out :P

1238.5 g water (H20) for 17.5 g of CuCl2

Can you find how many moles of water will be there in 1238.5g of H20?

1 mole of water = 18g

68.8 mol?

what do I do with this number?

right!!.......sry I was away

now find the number of CuCl2 moles in 17.5g of CuCl2

what is the number of CuCl2? Look up the atomic mass?

yeah u can do that atomic mass of Cu + 2* atomic mass of Cl

so 63.546 + 2 =127.092* 35.45 = 4,505.4114? right?

Nope

it seemed wronged

M(Cu) = 63.546 M(Cl) = 35.45 M(CuCl2) = M(Cu) + 2M(Cl)

so 63.546 + 70.9 =134.446?

right!!

go on find the number of moles in 17.5g of CuCl2

is it 7.682 /18? so .426? and thanks for all the help, by the way.

Nope 1 mole of CuCl2 is equal to the atomic mass of CuCl2 you calculate -> 134.446g So use simple unitary method to find how many moles will be there in 17.5g of CuCl2

134.46g of CuCl2 === 1mole -> 1g of CuCl2 === (1/134.46)mole so 17.5 g of CuCl2 === ??? moles

2,353.5 g?

134.46 * 17.5

nope

it is 17.5 *(1/134.46)

.00743

where do I go from here?

you still have your calculations wrong there!

anyway if you find the number of moles of water in the sample and then the number of moles of CuCl2 -> you can divide them both to get the mole ratio

hmm I keep getting that .000743 or .130

0.130 is right!!

Oh great! so divide .130 and 134.46?

I just observed that you have to find the mole ration of water and CuCl2 in the original sample..............SO wt of the sample b4 dehydration was - 589g and after dehydration was 17,5 In dehydration all water was lost = 589g -17.5g =571.5g so number of moles of water in ORIGINAL SAMPLE = 571.5/18 = 31.75

so divide 31.75 and 0.13

you have your answer there

244.2:1

Thanks for all the help!

right!!

you are welcome..........i hope u understood the concept

Yeah, itsdefinitely made more sense as I worked through it

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