GCSE OCR Gateway Chemistry C5.1 Making a standard solution

C5.1a To explain how the concentration of a solution in mol/dm3 is related to the mass of the solute and the volume of the solution by making standard solutions.

How to make a standard solution.

First thing you have to do is decide whether it's a primary standard or a secondary standard you need to make up. 

Primary Standards

Primary standards are used to determine the concentration of other less stable reagents used in volumetric analysis.

Compounds used to make primary standard solutions have therefore to have certain properties.

Features of a primary standard:

• Unreactive except with the reagent they standardise.

• Low hygroscopicity i.e. they don’t absorb water vapour from the air on standing for a long time on the prep–room shelf.

• High molar mass to reduce errors in weighing and calculations.

• Can be obtained in a very pure form.

• Preferably non–toxic.

• Obviously soluble in water.

• Lastly if possible they need to be cheap and readily available.

The primary standard used to standardise alkaline solutions such as sodium hydroxide is solid potassium hydrogen phthalate (C₈H₅O₄K).

Sodium carbonate solid (Na₂CO₃) is used to standardise acidic solutions such as hydrochloric acid.

Once the sodium hydroxide solution or hydrochloric acid solution has been standardised it becomes a secondary standard. 

An important reason to standardise sodium hydroxide solution is to do with the stability of these solutions. 

You’ll have seen this I think.  If left for any length of time in the lab or prep room, sodium hydroxide solution reacts with carbon dioxide in the air.  It forms sodium carbonate in the solution and you can often see this because it looks like flakes or a white solid in the bottom of the sodium hydroxide bottle.

2NaOH(aq)   +    CO₂(g)     ⟶     Na₂CO₃(s)   +    H₂O(l)

So let’s suppose you need to make a 0.1M standard solution of sodium carbonate.

You have available a 250ml volumetric flask, an electronic balance weighing preferably to 3 decimal places and a supply of distilled water.  Here’s what you can do:

First calculate the mass of sodium carbonate you need to weigh out to make 250ml of the 0.1M sodium carbonate solution. 

First, work out the molar mass of anhydrous Na₂CO₃: its 106 g/mol.

So a 1M solution contains 106g of sodium carbonate therefore to find what a 0.15M solution contains use:

so

so

But there are only 250ml or a quarter of a litre in the flask so we need to weigh out 15.9g/4 or 3.975g of sodium carbonate.

Second, transfer this mass of sodium carbonate into a weighing bottle weighing it to the nearest 0.001g.

Third, put 50ml distilled water in a 250ml beaker and carefully transfer the bulk of the sodium carbonate from the weighing bottle into the beaker.

Fourth, reweigh the weighing bottle.

Fifth, stir to dissolve the solid adding more water if needed.

Sixth, transfer the solution to the volumetric flask through the filter funnel.  Rinse the beaker well and make sure all the liquid goes into the flask.

Seventh, make the solution in the flask up to the mark using a dropping pipette.

Lastly, shake the flask thoroughly several times to mix the contents completely.

Possible Errors:

1. Not making up to the mark:

If you do not add sufficient water and make the solution up to the mark then the concentration of the solution will be higher than you expected it to be.  You will have less water than expected.

2. Spilling some of the solid in making the transfer:

If you spill some of the solid then the solution contains less solid than expected so its concentration is lower than expected.

Another approach to the calculation of the mass of solute:

If concentration is 0.15M then use

to calculate the number of moles of solute required so

Knowing the number of moles use

to calculate  the mass of solute required so

so

You’ll read about how to carry out a titration in my next post.