Question

6) (30 points) A sample of 3500 ppm iron is Strong Acid has to be diluted between 20 and 45 ppm. The method chosen was to dilute in three dilutions of 50 mL each. The first dilution is a 1/5, the second dilution is a 1/6 and the third dilution is a 1/4. Determine the amount of strong acid to add to tube 1 and how much 7000 pm solution to add to tube 1. Determine the amount from tube 1 and the amount of acid to put in tube 2. Determine the amount of acid and the amount from tube 2 to put into tube 3.ch Determine the concentration of each tube. Is it diluted to the proper range? ution 1 = 1/5

that the way the teacher gives it . it does not give more information. Some students did it but i dont know how ?

Answer 1

This is a bit confusing, since it talks about a sample with a concentration of 3500 ppm and then from a 7000 ppm solution. My guess is that you have to work with the second, sin it says it is the one added to tube 1.

The dilution in tube 1 is a 1/5 dilution, which means that the volume of 7000 ppm solution added is 1/5 of the total volume, the rest being acid. This means that 10 mL (1/5 of 50 mL) of 7000 ppm solution were added and then 40 mL of acid, to make up the 50 mL. The resulting concentration is given by the dilution formula:

$C_{final}=\frac{C_{original}\cdot V_{original}}{V_{final}}=\frac{7000ppm\cdot 10mL}{50mL}=1400\: ppm$

(Which is the same as dividing the original concentration by 5).

Second tube is a 1/6 dilution of the 1400 ppm solution. The volume of this solution is 1/6 of 50 mL (8.33 mL) and the rest (41.67 mL) is strong acid. The resulting concentration is 1/6 of the 1400 ppm original one:

$C_{final,2}=\frac{1400ppm}{6}=233.3\: ppm$

And finally, tube 3 is a 1/4 dilution of tube 4. This means we have to add 1/4 of 50 mL (12.5 mL) of the solution in tube 2 + 37.5 mL of acid, to obtain a final concentration of:

$C_{final,3}=\frac{233.3\: ppm}{4}=58.3\: ppm$.

If we wanted to dilute to a concentration in the 20-45 ppm range, the dilutions were not correct, since we are above it.

Please bear in mind that if the calculations were to be done with 3500 ppm as the initial concentration, the calculations would be exactly the same and you would get a concentration of 700 ppm for tube 1, a concentration of 116.7 ppm for tube 2 and a concentration of 29.2 for tube 3. The volumes we calculated would not change.