Question

Prelab Question #2: A student has weighed out 850 mg of 1-pentanol in 50 mL volumetric flask and prepared four standard solutions as described in this procedure. Calculate concentrations of 1-pentanol in mg/mL in each of these solutions.

Question:

Answer 1

According to the lab procedure ,

We have to make standard solution in these steps :

Step 1. Weigh accurately the dry 50 ml volumetric flask with cap.

Step 2. Pipette 1ml of 1-pentanol into flask.

Step 3. Weigh the flask with contents.

Step 4. Dilute with 2 propanol until solution become 50 ml

Step 5. Mixing and filling the 10 ml burette with this solution

Now, when we know that weight of flask with contents = 850mg ( 1-pentanol in 50 ml flask ) we already done the step 3

Now we have the 50 ml solution in which 850 mg is 1- pentanol rest is 2-propanol

Concentration of this bulk solution = 850mg / 50ml = 17 mg/ml

Now , we need to prepare 5 solution , according to the given conditions

Solution no. 1

Standard stock ( burette ) or the solution we prepared = 0.5 ml

Internal standard = 0.2 ml

Amount of 1-pentanol in mg in solution 1 = concentration of bulk solution * vol of standard solution

= 17 mg/ml * 0.5 ml = 8.5 mg

Net volume of solution = 0.5 + 0.2 = 0.7 ml

Concentration of 1- pentanol in solution 1 = 8.5 mg / 0.7 ml = 12.143 mg/ml (answer)

Similarly for other 4 solutions,

For solution 2:

Solution we prepared = 1.0 ml

Internal standard = 0.2 ml

Net volume of solution = 1.0 + 0.2 = 1.2 ml

Amount of 1-pentanol in solution 2 = 17 mg/ml * 1.0 ml = 17 mg

Concentration of 1-pentanol in solution 2 = 17mg/1.2ml = 14.166 mg/ml (answer)

Solution no. 3 :

Solution we prepared = 1.5 ml

Internal standard = 0.2 ml

Net volume of solution = 1.7 ml

Amount of 1-pentanol in solution 3 = 17 mg/ml * 1.5 ml = 25.5 mg

Concentration of 1-pentanol in solution 3 = 25.5 mg / 1.7 ml = 15 mg/ml ( answer )

Solution 4:

Solution we prepared = 2 ml

Internal standard = 0.2 ml

Net volume of solution = 2+0.2 = 2.2 ml

Amount of 1-pentanol in solution 4 = 17 mg/ml * 2 ml = 34 mg

Concentration of 1-pentanol in solution 4 = 34 mg / 2.2 ml = 15.45 mg/ml ( answer )

Solution 5 :

Solution we prepared = 0 ml

Internal standard = 0.2 ml

Concentration of 1- pentanol in solution = 0 mg / 0.2 ml = 0 mg/ml ( answer )