Review Questions in Laboratory Mathematics

Questions in Laboratory Mathematics

Laboratory Mathematics is a necessary part of the Clinical Chemistry section of a Clinical Laboratory. Although, reagents and substances now are commercially prepared, it is still crucial that a Medical Technologist or Laboratory Scientist knows how to prepare the most common solutions used in CC. The need for serial dilutions is also essential in any section of the laboratory.

DIRECTIONS: For each of the questions, there is only ONE correct answer. Select the BEST answer.

Questions in Laboratory Mathematics

Determine the molecular weights of the following substances: Atomic weights: H=1, Na=23, Cl=35.5, S=32, O=16, Ca=40, N=14

1. NaCl
2. H2SO4
3. CaCl2.H2O
4. HCl
5. HNO3

Choices: A. 36.5 B. 58.5 C. 63 D. 98 E. 129 F. 111

6. How many milliliters of 0.5 N hydrochloric acid is needed to prepare 250 milliliters of 0.1 N hydrochloric acid?

A. 50 milliliters
B. 200 milliliters
C. 20 milliliters
D. 60 milliliters
E. 25 milliliters

7. If you are to prepare 150 milliliters of normal saline solution, how much of the solute will you need?
A. 1.60 milliliters
B. 10 milliliters
C. 15 milliliters
D. 1.3 milliliters
E. 1.5 milliliters

8. How will you prepare a serum-NSS dilution of 1:25, if you have 0.50 milliliters of serum?
A. Add 0.50 milliliter of serum to 20 milliliters of normal saline solution.
B. Add 0.50 milliliters of serum to 25 milliliters of normal saline solution.
C. Add 0.50 milliliters of serum to 12 milliliters of normal saline solution.
D. Add 0.50 milliliters of serum to 24 milliliters of normal saline solution
E. None of the above

9. What is the weight of CaCl2 needed to prepare 1 liter of 2 Molar solution?
A. 111 grams
B. 55.5 grams
C. 222 grams
D. 200 grams
E. 75 grams

10. What is the Normality of a 0.5% HCl solution?
A. 1.37
B. 1.80
C. 18.25
D. 10.85
E. 0.18

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Answers to Question on Laboratory Mathematics

1. B- The atomic weights are: Na (sodium) = 23, and Cl (chloride) = 35.5

            23+35.5 = 58.5

2. D – The atomic weights are: H (hydrogen) = 1 (x 2) since you have 2 molecules, S (sulfur)=32, and O (oxygen) = 16 (x 4) since you have 4 molecules of oxygen.

            2+32+ 64 = 98    

3. E – The atomic weights are: Ca (calcium)=40, Cl(chloride)= 35.5 x 2, H= 1 x 2, O = 16

            40+71+2+16= 129

4. A – The atomic weights are: H = 1, Cl=35.5

            1+35.5 = 36.5

5. C – The atomic weights are: H=1, N (nitrogen) = 14, O (oxygen) = 16 x 3

            1+14+48 = 63

6. A – 50 milliliters; this is solved using the formula, C1V1=C2V2

            Where: C1- concentration 1 (original concentration)

                           V1 – volume 1 (original volume)

                            C2 – concentration 1 (final concentration)

                             V2 – volume 2 (final volume)

            Analyzing the problem, you can substitute the values now.

            (0.5)V1) = (0.1)(250)

            V1 = (0.1)(250)/0.5

            V1 = 50 milliliters of 0.5 N HCl    

 

7. D – 1.275 milliliters or 1.3; you can solve it using two methods. The first method is through ratio and proportion. Normal Saline Solution (NSS) is 0.85% NaCl; it is the solute. If you want to round it off, to 0.9% you can. But for this particular problem, we will use 0.85%. The 0.85% NaCl is 0.85 grams of NaCl diluted to 100 milliliters of distilled water up to the 100 mark of a volumetric flask. Since 0.85 grams is to 100, we’ll make this as the basis:

0.85/100 = x/150

Transposing the equation to get x, we come up with this:

X= 0.85 x 150/100

X = 1.275 or 1.3 milliliters

The second method is to use the percent formula, which is:

%=Weight in grams/Total volume of solution X 100.

Substituting the values:

85= W in g/150 x 100

Transpose to isolate W in g and you’ll come up with this formula:

W in g = 0.85 x 150/100 = 1.275 grams of NaCl

Tips to remember:

* When solving for percent solutions in relation to weight and volume, convert first the volume to milliliters if it is not in milliliters.

* If the volume is 100 milliliters, substitute the percent sign with the grams and that is already the weight of the solute.

Examples:

·         5% CaCl2 is actually prepared by weighing 5 grams of CaCl2 and dilute it with distilled water up to the 100 mark in a volumetric flask. The volumetric flask is the most accurate glassware in the preparation of solutions.

·         10% NaOH = 10 grams NaOH diluted with distilled water up to the 100 mark in a volumetric flask.

           

9. C – 222 grams; to determine the weight of a substance when preparing for a Molar solution, use the formula:

            W= DV x DM x MW

            Where:

            W – Weight in grams

            DV- Desired Volume (in liters)

            DM – Desired Molarity

            MW – Molecular Weight

            Substituting the values, we get:

            W= 1 liter x 2 M x 111

            W = 222 grams of CaCl2

10. A – 1.37 N; You can use the formula for Normality, which is N = (Volume of solute/Volume of solution/MW/f)/Volume of solution in liters, and the formula for percent (V/V), which is % = Volume of solute in milliliters/Total volume of solution in milliliters x 100.

With the first formula, the substitution would be: 

            N= ((0.5/100)/(36.5/1)/1

            N = (0.005)/36.5

            N = 1.3698 0r 1.37 

The volume of solution is assumed to be 100 milliliters because it is a percent solution.

You can also use the conversion units, making use of the percent formula. For Normality your units would be milliequivalent per liter of solution. (MEq/L)

Questions in Laboratory Mathematics

Determine the molecular weights of the following substances: Atomic weights: H=1, Na=23, Cl=35.5, S=32, O=16, Ca=40, N=14

1. NaCl                          
2. H₂SO₄
3. CaCl₂.H₂O
4. HCl
5. HNO₃

Choices:

            A. 36.5

            B. 58.5

            C. 63

            D. 98

            E. 129

F. 111

6. How many milliliters of 0.5 N hydrochloric acid is needed to prepare 250 milliliters of 0.1 N hydrochloric acid?

A. 50 milliliters

B. 200 milliliters

C. 20 milliliters

D. 60 milliliters

E. 25 milliliters

7. If you are to prepare 150 milliliters of normal saline solution, how much of the solute will you need?

A. 1.60 milliliters

B. 10 milliliters

C. 15 milliliters

D. 1.3 milliliters

E. 1.5 milliliters

8. How will you prepare a serum-NSS dilution of 1:25, if you have 0.50 milliliters of serum?

A. Add 0.50 milliliter of serum to 20 milliliters of normal saline solution.

B. Add 0.50 milliliters of serum to 25 milliliters of normal saline solution.

C. Add 0.50 milliliters of serum to 12 milliliters of normal saline solution.

E. Add 0.50 milliliters of serum to 24 milliliters of normal saline solution

9.  What is the weight of CaCl₂ needed to prepare 1 liter of 2 Molar solution?

A. 111 grams

B. 55.5 grams

C. 222 grams

D. 200 grams

E. 75 grams

10. What is the Normality of a 0.5% HCl solution?

A. 1.37

B. 1.80

C. 18.25

D. 10.85

E. 0.18

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