Understanding the concept of dilution to calculate the content of substance inside the sample
A. INTRODUCTION
Based on my experience during my college time, some of my colleagues were having a hard time calculating the molarity or moles of chemical substance, once
they involved in a series of sample preparation steps . The example of this preparation steps are described as follow :
“10 gram of sample that contain substance A is dissolved into 50 mL of water, then diluted to 100 mL with water. 10 mL of this solution is then diluted again to 1000
mL with water. Finally, 10 mL of this solution is tested by titration. “.
What is the content of substance A ( %w/w) in the sample? If you have difficulty in this kind of problem, then this note might help you.
B. METHODOLOGY
To solve this kind of problem, you can just imagine the state of the properties in each problem in your head. But, not all poeple has a good “imagination”,
therefore i offer a systematic approach to solve this dilution problem.
Step 1. Know your final target . If your final target is the concentration of substance in % w/w or ppm, then you need to do unit conversion after get the final mole.
Step 2. Breakdown the long sentence ( of preparation steps) into a numbered step by step preparation .
Step 3. Make sure each number do not contain more than 1 step ( e.g : a portion of 10 mL of sample is transfererred to volumetric flask, then add water until 100
mL ) . You should separate the steps into 2 steps instead : 1. a portion of 10 mL of sample is transferred to volumetric flask. 2. Add water until 100 mL. Why this step
important? Because if your numbered steps contain multiple steps on it, your state ( either mole or molarity ) will change at the same time, hence make your
calculation more difficult.
Step 4. Create a simple decision tree that describe the change in either concentration or mole of the chemical substance in each steps. The example is shown
below :
No Step If Yes If No
1 Is the sample is in liquid or solution form? a. you can calculate the molarity , otherwise you need to express the a. you can calculate the
concentration in other units, such as %(w/w) or ppm. molarity
b. You can calculate mole b. you can calculate mole
2 Is it dissolving process? a. you can calculate the molarity, just check the final volume of solution. If N/A
you dont know exactly the volume used to dissolve the sample, then you
Note ( dissolving and diluting is different !) can’t calculate the molarity.
b. The mole is unchanged
3 Is it dilution process ? ( change in volume in a. The molarity is changed N/A
current step ) b. The mole is unchanged
4 Is aliquot taken ? ( a portion of solution in current a. The molarity is unchanged N/A
step ) b. The mole is unchanged
Step 5. Create a step by step process and answer the question presented in Step 4
A. INTRODUCTION
Based on my experience during my college time, some of my colleagues were having a hard time calculating the molarity or moles of chemical substance, once
they involved in a series of sample preparation steps . The example of this preparation steps are described as follow :
“10 gram of sample that contain substance A is dissolved into 50 mL of water, then diluted to 100 mL with water. 10 mL of this solution is then diluted again to 1000
mL with water. Finally, 10 mL of this solution is tested by titration. “.
What is the content of substance A ( %w/w) in the sample? If you have difficulty in this kind of problem, then this note might help you.
B. METHODOLOGY
To solve this kind of problem, you can just imagine the state of the properties in each problem in your head. But, not all poeple has a good “imagination”,
therefore i offer a systematic approach to solve this dilution problem.
Step 1. Know your final target . If your final target is the concentration of substance in % w/w or ppm, then you need to do unit conversion after get the final mole.
Step 2. Breakdown the long sentence ( of preparation steps) into a numbered step by step preparation .
Step 3. Make sure each number do not contain more than 1 step ( e.g : a portion of 10 mL of sample is transfererred to volumetric flask, then add water until 100
mL ) . You should separate the steps into 2 steps instead : 1. a portion of 10 mL of sample is transferred to volumetric flask. 2. Add water until 100 mL. Why this step
important? Because if your numbered steps contain multiple steps on it, your state ( either mole or molarity ) will change at the same time, hence make your
calculation more difficult.
Step 4. Create a simple decision tree that describe the change in either concentration or mole of the chemical substance in each steps. The example is shown
below :
No Step If Yes If No
1 Is the sample is in liquid or solution form? a. you can calculate the molarity , otherwise you need to express the a. you can calculate the
concentration in other units, such as %(w/w) or ppm. molarity
b. You can calculate mole b. you can calculate mole
2 Is it dissolving process? a. you can calculate the molarity, just check the final volume of solution. If N/A
you dont know exactly the volume used to dissolve the sample, then you
Note ( dissolving and diluting is different !) can’t calculate the molarity.
b. The mole is unchanged
3 Is it dilution process ? ( change in volume in a. The molarity is changed N/A
current step ) b. The mole is unchanged
4 Is aliquot taken ? ( a portion of solution in current a. The molarity is unchanged N/A
step ) b. The mole is unchanged
Step 5. Create a step by step process and answer the question presented in Step 4
