Calculating Ca2+ Weight Percentage Using EDTA Titration Method | Lab Help

[higherme]

My question is:
Using the average [EDTA], find the weight% of Ca2+ in the unknown

My attempt:

so, the[EDTA] i found from standardizing was 0.005 M;
I used 29.85mL to of EDTA to titrate my 25.00mL unknown
and the weight of my unknown used was 0.2505 g

this is what i did:
[EDTA] = 0.005M * 0.02985L = 1.489E-4 mol EDTA = mol Ca2+

so that is the moles of Ca2+ , my question is do I have to convert this moles to moles in 250mL, because the unknown was prepared in a 250mL volumetric flask, and only 25.00mL was taken out from that flask for titration. Does that mean 1.489E-4 moles (calculated above) is only the amount of moles in 25.00mL??

any help appreciated :P

 

[chemisttree]

My question is:
Using the average [EDTA], find the weight% of Ca2+ in the unknown

My attempt:

so, the[EDTA] i found from standardizing was 0.005 M;
I used 29.85mL to of EDTA to titrate my 25.00mL unknown
and the weight of my unknown used was 0.2505 g

this is what i did:
[EDTA] = 0.005M * 0.02985L = 1.489E-4 mol EDTA = mol Ca2+

so that is the moles of Ca2+ , my question is do I have to convert this moles to moles in 250mL, because the unknown was prepared in a 250mL volumetric flask, and only 25.00mL was taken out from that flask for titration. Does that mean 1.489E-4 moles (calculated above) is only the amount of moles in 25.00mL??

any help appreciated :P

Of course!

 

[higherme]

so do i have to use m1v1=m2v2 to find the moles that is in 250mL

can i just use the number in moles for m1... or do i have to change it to concentration first

(1.489E-4 mol)(25.00mL) = m2 (250mL)
m2 = 1.489 E-5 moles in 250mL ?

this is the moles of Ca2+ in the unknown... i divide by the total weight of the unknown to find the % Ca2+

 

[chemisttree]

You are given the weight of the unknown as 0.2505. You dissolved this into 250 mL and analyzed a 25 mL aliquot of this unknown using 29.85 mL of 0.005 M EDTA.

You used 25 mL out of a solution of 250 mL. What fraction of 250 mL is 25 mL? That math is pretty simple.

 

[higherme]

so in 250mL i have 1.48E-4 mol

25 ml /250 ml = 0.1

0.1*1.48E-4 mol = 1.48E-5 mol... which will be the same as what i got using mv=mv

 

[chemisttree]

so in 250mL i have 1.48E-4 mol

25 ml /250 ml = 0.1

0.1*1.48E-4 mol = 1.48E-5 mol... which will be the same as what i got using mv=mv

Why are you doing this? Why did you DIVIDE by 10! Is there 1/10 as much in 250 mL as there is in 25 mL?

 

[higherme]

oh oops.. should be times 10... 1.48E-4 moles * 250/25 = 1.48E-3 moles