I haven\'t done these calculations in some time. I tried #1 and #2, I dont think
ID: 574242 • Letter: I
Question
I haven't done these calculations in some time. I tried #1 and #2, I dont think #2 is correct. Please help. The more explaination, the better. Thank you
1) The formula weight of NaCl is 58.44 g/mol. How would you make 300 mL of a 0.75 M solution of NaCl?
(VxCxMW) 0.3L x 0.75moles/L x 58.44g/mole = 13.149g
2) If your bottle of solid NaCl stated that the salt was only 90% pure, how much would you now have to weigh out to make 300 mL of a 0.75 M solution?
(VxCxMW) 0.3L x (0.90% x 0.75moles/L) x 58.44g/mole = ?
3) Starting with your 0.75 M solution of NaCl, how would you make 100 mL of a 25 mM solution of NaCl?
4) You are given a 10% (w/v) solution of NaCl – what is the concentration in Moles/L?
5) You have been given a solution of ATP of unknown concentration. You take 1 mL of the solution, and dilute to a final volume of 50 mL. The absorbance of this dilute solution at 259 nm is 0.570. What is the concentration of the original solution:
a) In mmoles/liter?
b) In mg/ml?
The Molar extinction coefficient of ATP is 15,400 l/mol cm.
The formula weight of ATP is 507.18 g/mole
Explanation / Answer
1.
300 mL of a 0.75 M solution
So, moles of NaCl = 0.75 M x 300 mL
= 0.75 M x 0.3 L
= 0.225 moles of NaCl
Molar mass of NaCl = 58.4 g/mol
So, 1 mole of NaCl = 58.4 g
or, 0.225 moles of NaCl = 0.225 x 58.4 g
= 13.14 g
So, you now have to weigh out 13.14 g of solid NaCl and dissolve in 300 mL of solvent to make a 0.75 M solution.
2.
300 mL of a 0.75 M solution
So, moles of NaCl = 0.75 M x 300 mL
= 0.75 M x 0.3 L
= 0.225 moles of NaCl
Molar mass of NaCl = 58.4 g/mol
So, 1 mole of NaCl = 58.4 g
or, 0.225 moles of NaCl = 0.225 x 58.4 g
= 13.14 g
Now, the solid NaCl is 90% pure, which means
90 g of NaCl is present in 100 g of the solid NaCl
or, 1 g of NaCl is present in (100/90) g of the solid NaCl
or, 13.14 g of NaCl is present in 13.14 x (100/90) g of the solid NaCl
or, 13.14 g of NaCl is present in 14.6 g of the solid NaCl
So, you now have to weigh out 14.6 g of solid NaCl and dissolve in 300 mL of solvent to make a 0.75 M solution.
3.
We know that
M1 V1 = M2 V2
or, 0.75 M x V1 = 25 mM x 100 mL
or, 0.75 M x V1 = 0.025 M x 100 mL
or, 0.75 x V1 = 2.5 mL
or, V1 = (2.5 / 0.75) mL = 3.33 mL
Hence, take 3.33 mL of 0.75 M NaCl and mark the water to 100 mL.
4.
10% (w/v) solution of NaCl means
10 g of NaCl in 100 mL of solution
or, 100 g of NaCl in 1000 mL of solution
Now,
Molar mass of NaCl = 58.4 g/mol
So, 58.4 g of NaCl = 1 mol
or, 1 g of NaCl = (1 / 58.4) mol
or, 100 g of NaCl = (100 / 58.4) mol = 1.71 mol
So, the concentration in Moles/L = 1.71 mol/L