Question

1. You have been asked to make a 1:5 dilution of 20mL of patient sample. The final volume should be 100mL. Which of the following outlines the correct way to make this dilution? For the other answer options, explain why they are incorrect. A. 10mL of patient sample added to 40mL of saline B. 10mL of patient sample added to 50mL of saline C. 20mL of patient sample added to 100mL of saline D. 20mL of patient sample added to 80mL of saline 2. How much of a 15% solution would you need to prepare 200 mL of a 1% working solution? 3. A working solution of glucose contains 15 mg/100 ml. To make this working solution, 1 ml of the stock glucose was added to 299 ml of diluent. What is the concentration (g/L) of the stock solution

Answer 1

Q1. By 1:5 dilution it is meant that if 5 parts of final solution is made, there will be 1 part of the solute. So, basically 1 part of the solute is mixed with 4 parts of the diluent. In other words, dilution is the ration of sample added to final volume prepared. Let's evaluate each option:

Option	Preparation	Incorrect/Correct	Reason
A	10 mL of patient sample added to 40 mL of saline	Incorrect	If 10 mL of sample is added to 40 mL of saline the total volume is (10+40)=50 mL. Althoufg the dilution here which is 10/50=1:5, which is what we were asked, but the final volume is not 100 mL.
B	10 mL of patient sample added to 50 mL of saline	Incorrect	Again the total volume is not 100 mL. It is (10+50)=60 mL. Moreover, the dilution is 10/60=1:6, not 1:5.
C	20 mL of patient sample added to 100 mL of saline	Incorrect	Total volume is 120 mL (20+100) while 100 mL was asked. The dilution ratio is 20/120= 1:6.
D	20 mL of patient sample added to 80 mL of saline	Correct	Total volume of final solution is what was asked, i.e. (20+80)=100 mL. The dilution is 20/100=1:5

Q2. The formula applicable here is c1v1=c2v2; where c1 and c2 are initial and final concentrations, respectively, and v1 and v2 are initial and final volumes respectively. Here, c1=15%, v1=? (say x); c2=1%, v2=200 mL.

Hence, 15*x= 1*200 or, x= 200/15=13.33

Thus, 13.33 mL of 15% solution will be needed.

Q3. Let's assume that the concentration of stock solution is x mg/ dL ( 1 dL= 100 mL). So, c1=x, v1=1 mL; c2= 15 mg/ dL, v2= 300 (final volume=diluent+added solution=1+299). Here c1 and v1 are concentration and volume of stock solution, respectively. c2 and v2 are concentration and volume of working solution.

So, c1v1=c2v2

or, x*1=15*300

or, x= 4500 mg/dL= 4.5 gm/dL=4.5 gm/100 mL=0.45 gm/Litre

Ans: The concentration of the stock solution is 0.45 gram per litre.