Analysis: Solving problems which involve in standard addition method formula

Question

A chemist determined the levels of iron in dry leaves collected from a plant near her homestead, she weighed 15 grams of the leaves and dissolved them in a mixture of acids, filtered the resulting solution in a 250mlvolumeric flask. She pipetted 10mls of the solution to a 50ml volumetric flask and topped it to the mark using deionized water. This solution gave absorbance of 0.56. She pipetted another 10mls into another 50ml flask, added 4mls of 3 x 10^^-4 M iron solution and made it to the mark with deionized water. This solution gave absorbance of 0.72.

(i)                  Calculate the concentration of iron in prepared leaf solution?

(ii)                Calculate the concentration of iron in the leaves in mg/g of the leaves?

This question needs the application of the formula of “standard addition method’’. The formula is given by:

SSVf/ ((Cx Vx) + (CsVs)) = SVf/ (CxVx);

Where:

SS- represent absorbance of the spiked sample

S-Represent absorbance of unspiked sample

Vf-represent final volume of either spiked sample or unspiked sample

Cx- represents concentration of the sample

Vx- represents the volume of the sample 

Cs-represent the concentration of the standard

Vs-represent the volume of the standard

NB: spiked sample is a sample in which the standard has been added to while unspiked one is that without addition of the standard.

Solutions

(i)

The following are provided:

SS=0.72

S=0.56

Vf=50ml

Vx= 10ml

Vs= 4mls

Cs= 3 x 10^^-4

NB: moles always confuse most students, don’t do this:

1000mls = 3 X 10^^-4

3mls =?

The molarity in this case doesn’t change.

Since the information provided is enough to get the concentration of the species under investigation. Substitution in the above equation will give the answer as shown below:

0.72 X 50/((Cx X 10ml) + (3 X 10^^-4 X 4ml)) = 0.56 X 50/(Cx X 10ml)

36(Cx X 10ml) =28((Cx X 10) + (3 X 10^^-4 X 4ml))

 360Cx= ((280Cx +28(0.0012))

 360Cx = 280Cx +0.0336

Collecting like terms;

360CX – 280CX = 0.0336

80Cx = 0.0336

Cx= (0.0336/80)

=0.00042 moles

(ii)

The mass of iron in 10mls of sample will be;

R.A.M of iron = 55.845g

But to get number of moles of substance is given by:

Given mass/R.A.M of that substance

Mass of iron in 10ml = (0.00042 X 55.845)g

= 0.02345g

But answer is needed in miligrams, therefore converting it gives:

(0.02345g X 100)

= 2.345 miligrams

Now concentration of iron in 250ml of prepared solution gives:

10ml = 2.345mg

250ml=?

(250 X 2.345mg)/10

=586.25mg

In 15 grams of leaves which were weighed, this become: Divide the answer by 15 grams;

(586.25mg/15g)

= 39.08333mg/g