Preparation of a standard solution of sodium carbonate
A standard solution is any solution in which we know the exact concentration. A standard solution of a solid can be prepared by weighing a mass of the solid, and dissolving it in a known volume of solution in a volumetric flask. Today, you will prepare a standard solution of sodium hydroxide. Since the dissolution process is exothermic, we will be using this solution in a second lab next class.
A standard solution is made by dissolving an accurate mass of solute into a known volume of water. The first step is to calculate the mass of solute required to make up a standard solution. For
example, if asked to prepare 250 cm3 (0.25 dm3) of a 0.100 mol dm−3 sodium carbonate solution, you must first calculate what mass of sodium carbonate you need to weigh out.
In equations used for calculating amounts and concentrations, the symbols refer to the following quantities:
C = concentration (mol dm−3)
n = number of moles
V = volume (dm3) m = mass (g)
Mr = molar mass (g mol−1)
Please note that not all substances make good standard solutions. This is due to the fact that some substances can be difficult to obtain in a completely pure form, are unstable in air or are not readily soluble in water.
Part 1: Calculating the mass of solute required
Show the formulas you used and the calculations with the correct Sig Fig.
Part 2: Making 250 cm3 of a standard solution
YOU WILL NEED
• top-pan balance and weighing boat
• 250 cm3 beaker
• glass or plastic stirring rod
• filter funnel
• plastic dropper for delivering small volumes
• 250 cm3 volumetric flask
• eye protection
• distilled water in a wash bottle • solute
You must make sure the volume of solution in the volumetric flask doe not go over the mar on the
neck of the volumetric flask.
- Use the weighing boat to weigh out the required amount of solute. Empty it into 250 cm3 beaker.
To ensure there is no solute remaining in the weighing boat, wash the weighing boat
twice using distilled water from a wash bottle and pour the washings into the beaker.
- Add more water to the beaker so that you have about 100 cm3.
Stir the mixture with the stirring rod until all the sodium carbonate has dissolved.
- Place the filter funnel into the neck of the 250 cm3 volumetric flask and pour the contents of the
beaker into the flask.
- Using a wash bottle, rinse the beaker and pour the washings into the volumetric flask. Repeat
this several times. You must also rinse the stirring rod.
- When the level of the liquid is just a few cm3 below the mark on the neck of the volumetric flask,
take the dropper and with great care use it to add distilled water from the wash bottle to the
solution one drop at a time until the lowest point of the meniscus is touching the line, as shown
in Figure belowTIP
If you go over the mark and the level of the liquid is above the line then you must reweigh your
solute and repeat the preparation of the solution.
- Place the stopper in the neck of the volumetric flask and, keeping the stopper firmly in the neck
using your thumb, mix the solution by turning the flask upside down at least five or six times
(see Figure P.3). If you move the flask and still see swirling currents in the liquid you have not
mixed enough-just turn upside down a few more times.
Post Lab Questions:
- Why do you have to prepare the solution in advance?
- How many grams of NaOH did you use for the solution?
- How many moles of solute were present in the volume prepared?
- How many mL of solution did you prepare?
- Calculate the Molarity of the solution in # moles of NaOH/dm3