Please explain how to prepare 1000ml of a 0.1 Molar solution of NaOH. The molecular weight of NaOH is 40.0g.

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t-nez eNotes educator| Certified Educator

A 0.1 M solution contains one mole of solute per liter of solution. Begin by calculating the mass of solute needed for the desired volume and concentration. In this case you need to prepare 1 liter (1000 ml), so you need 0.1 moles of NaOH. When you're preparing a volume other than one liter and the amount of solute isn't as obvious, it can be calculated using moles = molarity x volume (in liters).

You will need to convert 0.1 moles of NaOH to grams so you can measure it by weighing. The molar mass of NaOH is the mass of one mole.

0.1 moles NaOH x 40.0g/1 mole = 4 grams NaOH needed

Now that you have the amount of solute, here's the correct procedure:

1. Wear goggles and an apron because NaOH is caustic.

2. Place a weighing boat or weighing paper on a balance and add NaOH until the balance reads 4 grams. If you're using weighing paper, first fold it diaonally both ways and unfold leaving a creased "x". This makes it easier to empty the NaOH into a flask.

3. Pour the solid NaOH into a 1000 ml volumetric flask using a funnel and bending the weighing boat or paper diagonally. The flask will have a line on the neck that represents 1000 ml. Fill it up to about 2 cm below the line, using distilled or dionized water. Put the cap on the flask, invert it and swirl gently. Add more water, drop by drop, until the meniscus (low point) of the water is at the line. Repeat the mixing procedure and check the volume again. Add more water if needed to get 1000 ml of solution. 

The reason for not filling to the line initially is that the combined volume of the solute and solvent can be less or more than the volume before mixing. NaOH will tend to have a lower volume after mixing.

 

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