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It is often necessary for a chemist to understand how to make chemical solutions to the appropriate concentration. This paper describes how a chemist can make 500 grams of a 5% aqueous hydrochloric acid (HCl) solution from a 37% (by mass) HCl stock solution.
To prepare 500 grams of a 5% aqueous HCl solution, the chemist must have some basic information about the solute and the solution. The chemist must first determine the amount of solvent and solute needed to make the solution. In particular, the chemist must determine how many liters of solvent (water) and moles of solutes (HCl) are needed to make the solution (Hill & Finster, 2010, p.90). It is worth noting that the concentration of a solution is the number of moles of a substance divided by the number of liters of the solution.
It is essential to take safety precautions when dealing with potentially harmful substances like concentrated HCl. The safety precautions to take while making the aqueous HCl solution include wearing protective gloves, a lab coat, and an eye protector (Nielsen, 2010, p.98). It is also important that the solvent, in this case, water, is added first before HCl is added. Adding water to concentrated HCl solution often results in a violent reaction (Hill & Finster, 2010, p.5-4), which can cause harm to the person preparing the solution. The HCl acid should also be handled carefully. In addition, an acid-resistant fume-hood should be used to carry out the mixing. In the case of acid spilling on the skin, the victim should wash the skin with a lot of water.
If the chemist requires 500 grams of a 5% HCl solution, the mass of HCl needed is as shown below. Note that,
- HCl / (HCl + H2O) 5% = 0.05grams, and
- HCl + H2O => 500 grams.
Therefore, the amount of pure HCl needed is 25grams (500g * 0.05g), while the amount of water needed is 475 grams (500g – 25g). However, since the stock solution contains some water, the amount of water needed must be less than 475 grams.
Since the HCl stock solution is 37% by mass and contains some water, HCl / (HCl + H2O) is 0.37. The amount of solution to take from the stock solution so that we can have only 25 grams of HCl is determined as follows. Let s be the hydrochloric acid (HCl) and water (H2O) mixture.
25 / s = 0.37
s = 25/0.37 = 67.57g
Therefore, we should get 67.57 grams of the HCl and H2O mixture from the HCl stock solution. The amount of water in the 67.57g solution is 42.57 grams (67.57g – 25g). Since we have 25 grams of HCl and 42.57 grams of water from the stock solution, we only need 432.43 grams (475g – 42.57g) of additional water to make 500 grams of the 5% HCl solution.
Mixing HCl and water
After taking the necessary precaution and knowing the amount of solvent (water) and solutes (HCl) needed, the next step is to do the actual mixing of HCl and water. The equipments and tools needed include a fume-hood, a volumetric flask, a beaker, and a glass funnel. The steps involved are as follows:
- Ensure the fume-hood is efficiently ventilated and operating properly.
- Add 432.43 grams of distilled/deionized water into a dry, empty, and clean volumetric flask and place the flask in the fume-hood.
- Pour an appropriate amount of the stock HCl acid, precisely 67.57 grams, into the beaker using a glass funnel.
- Pour from the beaker about half of the desired HCl into the volumetric flask through a glass funnel and place the lid firmly on the flask.
- Swirl the flask gently to mix the solution.
- Open the volumetric flask, add the remaining HCl from the beaker into the volumetric flask place the lid back tightly on the flask.
- Invert the flask gently to properly mix the solution and make it homogenous. The resultant solution is 500 grams of a 5% HCl solution.
Hill, R. H. & Finster, D. (2010). Laboratory safety for chemistry students. Hoboken, New Jersey: John Wiley and Sons.
Nielsen, S. S. (2010). Food Analysis Laboratory Manual. London: Springer.