BSI Education

Practice Assignment 07

Method for preparing a titration reagent

Chemists often have to make up a standard solution. It is a solution with a precise concentration. Standard solutions are used in titrations. One of these is sodium chloride solution. This can be used to standardise silver nitrate solution - in other words, to determine its exact concentration. You will use standardised silver nitrate solution if you do experiment PA 08: A better bit of butter.

You are going to prepare a 0.025 mol dm-3 solution of sodium chloride. Your teacher will first demonstrate the procedure, then it will be your turn - so watch carefully.

Remember: you will need to be careful, accurate and methodical if you are to be successful.

What you have to do:

Note: Remember to always use distilled water

  1. Get a copy of Standard Procedure SP 0007:2005, and read it carefully. The chemicals and amounts used below are different, but the basic method is the same for any standard solution.
  2. Weigh out between 0.35 g and 0.40 g of sodium chloride into a weighing bottle. This is a very small amount (a pinch of salt), so do it carefully.
  3. Record the exact mass of sodium chloride you have weighed out.
  4. Carefully pour the weighed sodium chloride into your 250 cm3 beaker.
  5. Wash out the weighing bottle into the beaker, using distilled water.
  6. Add a further 50 cm3 distilled water, using a measuring cylinder.
  7. Stir the solution in the beaker using a glass rod until the sodium chloride has all dissolved.
  8. Pour this solution through a funnel into your 250 cm3 volumetric flask.
  9. Rinse the beaker, glass rod and funnel into the flask with a further small portion of distilled water.
  10. Make up to the mark on the volumetric flask. That means add distilled water until the bottom of the meniscus is level with the graduation mark on the neck.
  11. Stopper your flask and mix it well by inverting ten times, while holding the stopper in with your thumb.

Note: You or your teacher may have time to titrate your solution to see how accurate you have been.

Some hints

  • It is best to make up to the mark at eye level, so bend down to do it.
  • Use the dropper to add the last few drops of distilled water, one at a time.
  • Remember, you can always add a bit more but taking some out is difficult and will make your result inaccurate.

Some questions:

Don’t forget to write up your results in your laboratory notebook.

  1. Calculate the exact concentration of your sodium chloride solution, using the following formula:
    concentration (in mol dm-3) = mass of sodium chloride x 4
                                                                 58.5
    This gives the number of moles of sodium chloride in 1 dm-3 (1000 cm3 or 1 litre). We multiply by 4 because the mass is the amount you dissolved in 250 cm3. The mass in 1 dm-3 would be 4 times as much. We divide by 58.5 as this is the mass of one mole of sodium chloride.
    Label the flask with your name and “x mol dm-3 sodium chloride” (where "x" is the exact concentration you have just worked out).
    Note: "x" should be between 0.0239 and 0.0274. Provided the exact concentration is known, it does not need to be precisely 0.025 mol dm-3.
  2. Explain why the following parts of the procedure are necessary:

    a. using distilled water;
    b. washing out the weighing bottle into the beaker, and rinsing the beaker, glass rod and funnel into the flask;
    c. getting the bottom of the meniscus level with the graduation mark;
    d. inverting the flask several times.

Useful links

Here are some places you might want to look for further information:

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