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4.3. How to prepare 0.1 M hydrochloric acid?

Hydrochloric acid is the aqueous solution of hydrogen chloride gas (HCl). Concentrated hydrochloric acid (36-38%) is a colorless to yellowish pungent liquid. It is a strong irritant to eye and skin and fumes are harmfull if inhaled.

HCl solutions are usually prepard by dilution of the concentrated solution. The molarity of the concentrated solution can be calculated from the knowledge of density of the solution and percentage of HCl. 36.5 % hydrochloric acid means that 100 grams of this acid contains 36.5 grams of HCl.

Problem: Calculate the volume of concentrated HCl, having a density of 1181 g/L and containing 36.5 % HCl by weight, needed to prepare 500 mL of a 0.1 mol/L hydrochloric acid solution?

SolCalc: Preparing 0.1 M HCl

                    == SolCalc - Laboratory Report ==
                      September 03, 2012  13:20:20

HYDROCHLORIC ACID
c(HCl) = 0.1 mol/L
=======================
To prepare 500 mL of a 0.1 mol/L solution of hydrochloric acid we will need to
dilute 4.22917 mL of 36.5 % HCl to a final volume of 500 mL with deionized
(distilled) water.

SOURCE:
-------------
    Name: Hydrochloric acid  
    Formula: HCl
    Formula weight: 36.461 g/mol
    CAS Number: 7647-01-0 
    NFPA: Health 3, Flammability 0, Instability 1, Special 

CALCULATION:
-------------
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 = 500 mL * (1 L)/(1000 mL) = 0.5 L
    c0 = 0.1 mol/L
    Source solutions:
    w1 = 36.5 % = 36.5 * 1/100 = 0.365
    d1 = 1181 g/L
    --------------------------------

Then, we determine the concentration of the source (stock) solution

    c1 = d(HCl) * w(HCl) / M(HCl)
    c1 = 1181 g/L * 0.365 / 36.4609 g/mol
    c1 = 11.8227 mol/L

Since the total amount of solute is the same before and after dilution, the
volume of stock solution needed is

    V1 = V0 * c0 / c1
    V1 = 0.5 L * 0.1 mol/L / 11.8227 mol/L
    V1 = 0.00422917 L

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

    V(36.5 % HCl) = 0.00422917 L * (1000 mL)/(1 L) = 4.22917 mL

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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