Question

Using atomic absorption, 0.0105 grams of copper sulfate pentahydrate were dissolved in a 100 mL volumetric flask and diluted to mark to make a 100 ug/mL solution. An unknown was made by placing an unknown amount of the above stock solution in a 100 mL volumetric flask and diluting to mark. The unknown gave an absorbance of 5.809 mg/L. How many mL of stock was used to make this unknown? The answer should be between 1 and 10 mL

Answer 1

Concentration of stock solution 100 as said in the question.

Say, x mL of stock solution was taken.

Since the absorbance is given corresponding to the concentration 5.809 mg/L. So, it is the concentration of the final solution. Volume of the final solution = 100 mL.

We know, from the condition of equivalency,

V = volume and C = concentration.

Now, V_initial = x mL, C_initial = 100 mg/L, V_final = 100 mL and C_final = 5.809 mg/L

So, 5.809 mL of stock solution was taken.

Though, the concentration of the stock solution is not exactly 100

it is actually, 105 . If we carry out using that value, then the result becomes, 5.532 mL. The concentration of stock is defined in the question, so, we ignored this fact.