Question

Question 2 Expressing concentration in different units You have a 0.2 M solution of a compound (molecular weight 200) a) How many mmoles are there in the following volumes? i. 2 mL ii. 5 mL ii. 10 mL iv. 100 mL b) Express this concentration in the following units: i. mM; iii. iv. %(w/v); mg/dL; c) How much of this compound would you have to weigh out if you were to make up 100 mL of the solution? d) If you added 5 μL of this solution (0.2 M) to a PCR (polymerase chain reaction) with a final volume of 100 μL (2 marks) (i) How many moles would the reaction contain? (ii) What is the final concentration of this compound in the PCR assay?

Answer 1

a) To calculate the number of moles the formula is

n=CV

where C is concentration and V is volume'

i) n = 0.2M*2ml

= 0.4

ii)n = 0.2* 5

=1

iii) n= 0.2*10= 2

iv) n = 0.2*100 = 20

b) 0.2M into mM

0.2 x 1000 = 200mM

c) x grams = 0.2 M* 100ml * 200g/mol/ 1000

= 20*0.2

= 4g should be weighed to make 100ml of 0.2M solution

d) number of moles = 0.005 ml * 0.2

0.01 moles

final concentration would be

C1V1=C2V2

0.2M*5ul = x*100ul

X = 1/100 = 0.001 M