Help! Mole concentrations (1 Viewer)

[oyuck]

Hello All
I've been having horrible experiences with this question: can someone please assist?

Tube 4
0.200mol/L Fe(NO3)3 ------ 0.50mL
0.002mol/L KSCN -----------5.00mL
Water Added ---------------4.50mL

Calculate the effective dilution of these solutions after mixing but before reaction.
Work out:-

Initial [Fe3+] in mol/L

and

Initial [SCN-] in mol/L

I keep on getting the wrong answers which are
9.12 x 10^-3
and 0.12 x 10^-3
respectively

Working Please!! Thanks heaps !!

 

[nightweaver066]

First calculate the number of moles of Fe3+ you have.

Concentration = 0.200mol/L where the volume is 0.50mL
So the mole content = (0.200) x (0.0005) (in litres)
= 1 x 10^-4

Now looking at the formula, Fe(NO3)3, there is one mole of Fe3+ ions so there is (1 x 10^-4) mole of Fe3+ ions.

Now calculating the concentration of this with the added volume,
C = (1 x 10^-4)/(0.0005 + 0.005 + 0.0045)
= 0.01M or 1 x 10^-2 mol/L

For SCN-, do the same process.

Concentration of KSCN = 0.002mol/L and the volume is 5mL
Calculate number of moles: 0.002 x 0.005 (in litres)
= 1 x 10^-5

The formula is KSCN so the mole ratio between K+ to SCN- is 1:1 therefore the mole of SCN- is 1 x 10^-5

Dividing number of moles by volume to find concentration.
(1 x 10^-5) / (0.0005 + 0.005 + 0.0045)
= 1 x 10^-3M or 1 x 10^-3 mol/L

 

[oyuck]

First calculate the number of moles of Fe3+ you have.

Concentration = 0.200mol/L where the volume is 0.50mL
So the mole content = (0.200) x (0.0005) (in litres)
= 1 x 10^-4

Now looking at the formula, Fe(NO3)3, there is one mole of Fe3+ ions so there is (1 x 10^-4) mole of Fe3+ ions.

Now calculating the concentration of this with the added volume,
C = (1 x 10^-4)/(0.0005 + 0.005 + 0.0045)
= 0.01M or 1 x 10^-2 mol/L

For SCN-, do the same process.

Concentration of KSCN = 0.002mol/L and the volume is 5mL
Calculate number of moles: 0.002 x 0.005 (in litres)
= 1 x 10^-5

The formula is KSCN so the mole ratio between K+ to SCN- is 1:1 therefore the mole of SCN- is 1 x 10^-5

Dividing number of moles by volume to find concentration.
(1 x 10^-5) / (0.0005 + 0.005 + 0.0045)
= 1 x 10^-3M or 1 x 10^-3 mol/L

Thank you so much!

If I had to find the concentration of [NO3] would I have to multiply
1 x 10^-5 by 3
then divide by the total volume?

 

[nightweaver066]

Thank you so much!

If I had to find the concentration of [NO3] would I have to multiply
1 x 10^-5 by 3
then divide by the total volume?

Yes, multiply (1 x 10^-4) by 3 then divide it by total volume to find concentration of NO3-