Question

A student set up a standard plate count on a sample of milk in the following way: One ml of the milk was diluted into 99 ml of diluent. The first dilution was mixed well. With a new pipette, 0.1 ml of the first dilution was diluted into 0.9 ml of diluent. The second dilution was mixed well, and 0.1 ml of this second dilution was plated on a NA plate. What is the dilution factor of the plate?

A.) 10^-7

B.) 10^7

C.) 5 x 10^-4

D.) 10^-4

E.) 10^4

Answer 1

Dilution factor = Volume of resulting solution (milk +diluent)/ volume of milk

First dilution

1 ml of milk was diluted into 99 ml of diluent = so 1 ml of milk in 100 ml of resulting solution

Dilution factor 100/1= 10²

Second dilution

0.1 ml of the diluted solution is mixed with 0.9 ml of diluent. so 0.1 ml of diluted milk in 10 ml of resulting solution

Dilution factor = (initial dilution factor) X 10/0.1 = 10² X 10² = 10 ⁴

Answer E) 10 ⁴