SAMPLE LAB REPORT

4.2. How to mix two ammonia solutions?

Ammonia is a colourless gas with a characteristic irritating and pungent odour. This gas is toxic for humans and the environment. Ammonia is highly soluble in water. When it dissolves it produces a corrosive, alkaline solution (ammonia water or ammonium hydroxide) with a pH greater than 11.

Be careful: 28 % ammonia (NH₃) is equal to approximately 56.6 % ammonium hydroxide (NH₄OH).

When we mix two ammonia solutions which have different concentrations, the concentration of the final solution lies between the original concentrations. The amount of ammonia in the combined solution will be the sum of the amounts of ammonia in the constituent solutions.

Problem: A chemist has one solution that is 28 % ammonia and another solution that is 10 % ammonia. How many gallons of each should be mixed to get 3 gallons of a solution that is 20 % ammonia?

                    == SolCalc - Laboratory Report ==
                      September 03, 2012  13:10:07

AMMONIA
w(NH3) = 20 %
=======================
To prepare 3 gal of a 20 % solution of ammonia we will need to mix 1.71169 gal
of 28 % NH3 and 1.28831 gal 10 % NH3 and dilute the solution to a final volume
of 3 gal with deionized (distilled) water.

SOURCE:
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    Name: Ammonia  
    Formula: NH3
    Formula weight: 17.031 g/mol
    CAS Number: 1336-21-6 
    NFPA: Health 3, Flammability 0, Instability 0, Special 

CALCULATION:
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The key concept is that the amount of solute in the desired solution must be
equal to the amount of solute in the source solution. Remember, the
concentration is the amount of a solute divided by the volume of the solution.

Before we make any calculations we have to make sure that we only use one
system and one unit of measurement. DO NOT mix measurement systems and units.

    Desired solution:
    V0 = 3 gal * (1 L)/(0.264172 gal) = 11.3562 L
    w0 = 20 % = 20 * 1/100 = 0.2
    d0 = 7.70171 lb/gal * (1 g/L)/(0.00834541 lb/gal) = 922.868 g/L
    Source solutions:
    w1 = 28 % = 28 * 1/100 = 0.28
    d1 = 7.49381 lb/gal * (1 g/L)/(0.00834541 lb/gal) = 897.957 g/L
    w2 = 10 % = 10 * 1/100 = 0.1
    d2 = 7.99061 lb/gal * (1 g/L)/(0.00834541 lb/gal) = 957.487 g/L
    --------------------------------

First, we calculate the concentration of the desired solution

    c0 = d(solution) * w(NH3) / M(NH3)
    c0 = 922.868 g/L * 0.2 / 17.0305 g/mol
    c0 = 10.8378 mol/L

Then, we determine the concentration of the source solutions

    c1 = d(NH3) * w(NH3) / M(NH3)
    c1 = 897.957 g/L * 0.28 / 17.0305 g/mol
    c1 = 14.7634 mol/L
	
    c2 = d(NH3) * w(NH3) / M(NH3)
    c2 = 957.487 g/L * 0.1 / 17.0305 g/mol
    c2 = 5.62218 mol/L

When the two solutions are mixed, the final concentration of the resulting
solution can be determined by using the formula

    c0 * V0 = c1 * V1 + c2 * V2

By substituting V2 = V0-V1 into the above equation, we obtain

    V1 = (c0 - c2) / (c1 - c2) * V0
    V1 = (10.8378 mol/L - 5.62218 mol/L) / (14.7634 mol/L - 5.62218 mol/L) *
11.3562 L
    V1 = 6.47946 L
    V2 = 11.3562 L - 6.47946 L
    V2 = 4.87678 L

To convert the result into a desired unit we will use dimensional analysis
again

    V(28 % NH3) = 6.47946 L * (0.264172 gal)/(1 L) = 1.71169 gal
    V(10 % NH3) = 4.87678 L * (0.264172 gal)/(1 L) = 1.28831 gal

PROCEDURE:
-------------
First of all, fill the volumetric flask about halfway with deionized water to
avoid violent reactions. NEVER add water to concentrated acid.

Choose a clean pipette of suitable size and transfer the liquid to the
volumetric flask. When the whole solution has been drained, touch the tip of
the pipette to the side of the volumetric flask to allow the last of the liquid
to drain out. DO NOT blow out the remaining solution.

Allow the solution to reach room temperature because a volumetric flask is only
accurate at the temperature at which it has been calibrated (usually 20 °C).
Very carefully fill the flask to the mark on the neck of the flask, using a
dropping pipette to add the last few milliliters of liquid. Mix your solution
thoroughly, by inverting the flask and shaking. NEVER hold large volumetric
flasks by the neck alone - provide support at the bottom.

Transfer the prepared solution to a clean, dry storage bottle and label it.
NEVER store solutions in a volumetric flask.

SAFETY NOTES:
-------------
- When making chemical solutions, always use the appropriate safety equipment.
- As a general rule, always add the more concentrated solution to the less
concentrated solution.
- All chemicals that you are unfamiliar with should be treated with extreme
care and assumed to be highly flammable and toxic.

DISCLAIMER:
-------------
Use SolCalc at your own risk! If you don't understand the results, DON'T use
them.

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