Friday, July 12, 2013

Laboratory Math: Percent Solutions, Normality, Molarity Problems, Answers To Practice Problems




ANSWERS TO PRACTICE PROBLEMS:

1.       IF YOU HAVE WEIGHED 90 GRAMS OF NAOH AND DISSOLVED IT 1,500 ML OF WATER, WHAT IS THE;
A.      PERCENTAGE


% = Weight in grams  X 100
       Total Volume in mL

% = 90 x 100
       1,500

% = 6 % of NaOH solution


B.      MOLARITY

M= W in grams
        MW
         Liter of solution

M = 90
        40
        1.5

M = 1.5 M NaOH


C.      NORMALITY

N= Mv

N = 1.5 X 1

N = 1.5 N NaOH


2.       HOW MANY GRAMS DO YOU NEED TO PREPARE 2M OF NACL?

W = DM X DV X MW

W = 2 X 1 X 58.5

W = 117 grams of NaCl

How: weigh 117 grams of NaCl and dilute it with DH20 up to the 1 Liter mark in a volumetric flask or measure 500 mL of DH2O into a volumetric flask, weigh 117 grams of NaCl and dissolve. Add DH20 up to 1 liter mark.

3.       HOW WOULD YOU PREPARE 1N OF HCL?

C1V1 = C2V2

C1= 12 N (Concentrated HCl)

C2 = 1N

V2 = 100 mL (assumption when no volume is given)

V1 = unknown

V1 = C2V2
           C1

V1 = 8.33 mL

How: Add 8.33 mL of concentrated HCl to 91.67 mL of DH2O.


4.       HOW WOULD YOU PREPARE  0.85% NACL SOLUTION?

W = % X TV (total volume)
         100

W = 0.85 X 100 (if no volumes are given assume that TV is 100)
            100

W = 0.85 g (grams) of NaCl

How: Measure 50 mL of DH2O into a volumetric flask. Weigh 0.85 grams of NaCl and dissolve completely. Add DH2O up to the 100 mL mark.


5.       WHAT IS THE NORMALITY OF A 0.5 M CACL2 SOLUTION?

N = M X v (factor)

N = 0.5 X 2

N = 1 N CaCl2

6.       HOW DO YOU PREPARE A 1:10 SERUM DILUTION?

When no volume is given assume that 1 mL is one part; hence,

1 mL = 1 part of solute
9 mL = 9 parts of diluent
           10 parts (total)

Hence: dilution is 1:10

How: Add 1 mL of serum to 9 mL of NSS to come up with a 1:10 serum dilution.

7.       MAKE UP 500 ML OF 5M SOLUTION OF HCL


C1V1 = C2V2

V1 = C2V2
           C1

Before you substitute be sure that all your units are the same. It does not matter what units you used as long as they are similar units. i.e. if you use liters for volume all volumes should be expressed in liters.

V1 = 5 M X 500 mL
             12 (12 is standard Normal concentration of concentrated HCl, since valence is 1, N=M.)

V1 = 208.33 mL

How: Add 208.33 mL to 291.67 of DH2O to produce 500 mL of 5M HCl solution.

8.       PREPARE THE FOLLOWING WORKING STANDARD  SOLUTIONS FROM A STOCK SOLUTION OF 20 MG/DL: STATE VOLUME OF DILUENT AND DILUTION.

A.      15 MG/DL

                 C1V1 = C2V2

                 V1 = C2V2
                            C1
                V1 = 15 mg/Dl X 100 mL
                            20 mg/dL

                V1 = 75 mL 

75 mL of 20 mg/dL stock solution added to 25 mL of DH2O

B.      10 MG/DL

               C1V1 = C2V2

               V1 = C2V2
                          C1
                V1 = 10 mg/Dl X 100 mL
                            20 mg/dL

                V1 = 50 mL of 20 mg/dL stock solution added to 50 mL of DH2O

C.      5 MG/DL

              C1V1 = C2V2

               V1 = C2V2
                          C1

                V1 = 5 mg/Dl X 100 mL
                            20 mg/dL
                V1 = 25 mL of 20 mg/dL stock solution added to 75 mL of DH2O


D.      2 MG/DL

C1V1 = C2V2

V1 = C2V2
   C1

V1 = 2 mg/Dl X 100 mL
                            20 mg/dL

V1 = 10 mL of 20 mg/dL stock solution added to 90 mL of DH2O

Volume of stock solution, diluent and dilution are the following:
Volume of stock solution
Volume of Diluent (DH2O)
Dilution
75 mL
25 mL
1:4 (diluent to solute)
50 mL
50 mL
1:2
25 mL
75 mL
1:4
10 mL
90 mL
1:9

1.       75 divided by 25 = 3 + 1= 4, hence dilution is 1:4 (diluent to solute)
2.       50 divided by 50 = 1 + 1= 2, hence dilution is 1:2
3.       75 divided by 25 = 3 + 1= 4 hence dilution is 1:4
4.       90 divided by 10 = 8 + 1 = 9, hence dilution is 1:9



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