chemistry 2 question

Question

1. Assume that you have 1, 2, 5, and 10 mL pipets and a 25 mL volumetric flask only. Describe in detail how you would prepare a solution with a concentration near 0.100 M from a 0.650 M stock solution using the glassware listed above. Calculate the final concentration of the diluted solution (it won't be exactly 0.100 M). It should be within 10% of 0.100 M. (Please see the Lab Skills for the accuracy on volumetric pipets and flasks.)

Answer 1

To go from 0.650 M to 0.100 M represents a 6.5-fold dilution (0.650/0.100). Since the problem does not state the volume of 0.100 M to prepare, we could simply use this ratio and prepare, say, 10 mls as follows:

10 ml/6.5 = 1.54 mls of original

Use the 2 ml pipet to measure 1.5 mls of 0.650 M. Use the 10 ml pipet to add 8.5 mls to make 10 mls

Actual calculated concentration: (1.5 ml)(0.650 M) = (10 ml)(x M) and x = 0.0975 M = 0.098 M

If the question actually wants you to make 25 ml (using the volumetric flask), we can calculate the volume of the original 0.650 M to use:

(V1)(M1) = (V2)(M2)

V1 = initial volume = ?

M1 = initial molarity = 0.650 M

V2 = 25 ml

M2 = 0.100 M

(V1)(0.650 M) = (25 ml)(0.100 M)

V1 = 3.85 ml of 0.650 M needed

Use the 5 ml pipet to measure 3.9 mls, place it in the 25 ml volumetric flask and fill to the line.

Actual calculated concentration: (3.9 ml)(0.650 M) = (25 ml)(x M) and x = 0.101 M = 0.10 M