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Monday, January 2, 2017

Answers and Explanations to Laboratory Mathematics

Let’s check if your answers are correct to these Review Questions in Lab Math. If not, you can review the correct answers, and then take the test again. Understand the concept, so that when the values change, you still know the correct solution.

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.


•    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


    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.

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