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A) How do you make 200 mL of a 0.1 M solution of a substance that has a molecular weight of 121.1 g/mol?
Answer I got: 200mL = 1/0.1M x 1/(121.1g/mol) x mass of the substance.
mass of the substance = 0.2L x 0.1M x 121.1g/mol=2.42g
Note that my answer doesn't reveal how to do it?
B) If you take 10 mL of the solution you made in A, add 90 mL of water, mix and then take 5 mL of the mixture and bring it to 25 mL, what will be the concentration of the final solution in molars, millimolars, and micromolars?
Answer I used the multiple dilution equation first:
D1: 100/10= 10
D2: 25/5= 5
Df total: 5X10= 50 to 1
then I did this:
C2= C1/Df total:
C2= 0.1/50= .002 M;
.002/10^-3= 2 mM;
2/10^-6= 2 x 10^6 micromoles
5 answers
#1 has the correct number of grams. Having noted that the answer doesn't reveal how to do it, what do I do about it? Do you know how to make it up or do you want me to tell you how to make it up?
#2. M is ok at 0.002 M
That is 2 mM.
But it is 0.002 M x (1 M/10^6 micromols) = 2 x 10^-3?
Check my work.
#2. M is ok at 0.002 M
That is 2 mM.
But it is 0.002 M x (1 M/10^6 micromols) = 2 x 10^-3?
Check my work.
Would you inform me on how this is made up. I didn't know how to make it up once I found the grams or if I was even supposed to find grams. Secondly, isn't the last answer 2 x 10^3 because a micromole is 10^-6 so it would be .002M/10^-6= 2x10^3. Right?
Thanks
Thanks
You are correct on the micromolar. Note I didn't do the conversion properly. The units don't turn out right, either.
0.002 M x (1 x 10^6 micromoles/1 M) = 2000 micromoles.
How do we make the 0.1 M solution?
Take 2.42 g of the material, dissolve it in a little water, then make the final volume to 200 mL. Note that this is NOT the same thing as adding 2.42 g of the material to 200 mL water. In the first case, the final volume is 200 mL. In the second case, the final volume is more than 200 mL. In making molar solutions, one ALWAYS dissolves the substance in water, THEN make to some predetermined final volume.
0.002 M x (1 x 10^6 micromoles/1 M) = 2000 micromoles.
How do we make the 0.1 M solution?
Take 2.42 g of the material, dissolve it in a little water, then make the final volume to 200 mL. Note that this is NOT the same thing as adding 2.42 g of the material to 200 mL water. In the first case, the final volume is 200 mL. In the second case, the final volume is more than 200 mL. In making molar solutions, one ALWAYS dissolves the substance in water, THEN make to some predetermined final volume.
This is not correct because the second dilution factor should be (30mL/5mL)=6mL. The dilution factor is the total volume divided by the initial added. Therefore the final answer is .0017M, 1.7 millimolar, and 1.7x10^3 micromolar.
1 answer