Making a standard solution

Aim

The purpose of this experiment is to prepare a standard solution of potassium hydrogenphthalate.

Introduction

Potassium hydrogenphthalate, is a primary standard because it meets certain requirements.

• It must be available in a highly pure state.
• It must be stable in air.
• It must be easily soluble in water.
• It should have a high molar mass.
• In solution, when used in volumetric analysis, it must undergo complete and rapid reaction.

You will need to weigh accurately a sample of potassium hydrogenphthalate and use it to make a solution of concentration close to 0.10 mol dm³.

Requirements

• safety spectacles
• weighing boat
• spatula
• potassium hydrogenphthalate (IRRITANT)
• balance capable of weighing to within 0.01 g
• beaker, 250 cm³
• wash bottle of distilled water
• stirring rod
• volumetric flask, 250 cm³
• filter funnel
• dropping pipette

Procedure

1. Transfer between 4.8 and 5.4 g of potassium hydrogenphthalate into a weighing bottle and weigh it to the nearest 0.01 g.
2. Put about 50 cm³ of water into a 250 cm³ beaker. Carefully transfer the bulk of the potassiumhydrogen phthalate from the weighing boat into the beaker.
3. Reweigh the boat with any remaining potassium hydrogen phthalate to the nearest 0.01 g.
4. Stir to dissolve the solid, adding more water if necessary.
5. Transfer the solution to the volumetric flask through the filter funnel. Rinse the beaker well, making sure all liquid goes into the volumetric flask.
6. Add distilled water until the level is within about 1 cm of the mark on the neck of the flask. Insert the stopper and shake to mix the contents.
7. Using the dropping pipette, add enough water to bring the bottom of the meniscus to the mark. Insert the stopper and shake thoroughly ten times to ensure complete mixing. Simply inverting the flask once or twice does not mix the contents properly and is a very common fault.
8. Label the flask with the contents, your name and the date. Leave a space for the concentration to be filled in after you have calculated it. This can now be used to calculate the unknown concentration of a base.

Molar mass of potassium hydrogenphthalate,	g mol-1
Mass of bottle and contents before transfer, (m1)	g
Mass of bottle and contents after transfer, (m2)	g
Mass of potassium hydrogenphthalate, m = (m2 – m1)	g
Moles of potassium hydrogenphthalate, n = m/M	mol
Volume of solution, V	dm3
Concentration of potassium hydrogenphthalate, c = n/V	mol dm-3

Questions

What effect would each of the errors described below have on the concentration of potassium hydrogenphthalate?

(a) Some of the solid potassium hydrogenphthalate was spilled in making the transfer.

(b) Not enough water was added to bring the volume up to the mark.