Making Standard Solutions

Quantitative Chemistry

What is a standard solution?

A solution whose concentration is accurately known.

What is a primary standard?

A substance that can be accurately weighed and used to prepare a standard solution of known concentration.

What properties should a primary standard have?

It should have a high molar mass, high purity, be chemically stable, not absorb water from the atmosphere, be soluble in water and react rapidly and completely.

Why is a high molar mass desirable for a primary standard?

It reduces the percentage uncertainty in the measured mass.

Why must a primary standard have a high degree of purity?

So the measured mass accurately represents the amount of the required substance.

Why must a primary standard be chemically stable?

So it does not decompose or react with substances in the air.

Why should a primary standard not absorb water from the atmosphere?

Absorbed water would increase its measured mass and make the calculated number of moles inaccurate.

Why must a primary standard be soluble in water?

So it can completely dissolve when preparing the standard solution.

Why should a primary standard react rapidly and completely?

So its reactions in titrations have a known and reliable stoichiometry.

Why is hydrochloric acid unsuitable as a primary standard?

It does not exist as a solid that can be accurately weighed; it is used as an aqueous solution.

Why is sodium hydroxide unsuitable as a primary standard?

It absorbs water vapour and reacts with carbon dioxide from the atmosphere.

What can form when sodium hydroxide reacts with carbon dioxide in the air?

Sodium carbonate.

Why can sodium hydroxide not be accurately weighed as a pure substance?

It may contain unknown amounts of absorbed water and sodium carbonate.

What is an example of a primary standard suitable for acid-base titrations?

Sulfamic acid.

What is the formula of sulfamic acid?

NH₂SO₃H.

What is the molar mass of sulfamic acid?

97.1 g mol⁻¹.

What equation is used to calculate moles from concentration and volume?

n = cV.

What equation is used to calculate mass from moles and molar mass?

m = nM.

What units should volume be in when using n = cV?

dm³.

How do you convert cm³ to dm³?

Divide by 1000.

What volume in dm³ is 250 cm³?

0.250 dm³.

What apparatus is needed to prepare a standard solution?

Safety glasses, lab coat, accurate balance, weighing bottle or boat, spatula, beaker, volumetric flask, wash bottle of deionised water, small funnel and glass stirring rod.

Why is a volumetric flask used when preparing a standard solution?

It allows the solution to be made up accurately to a fixed known volume.

What is weighing by difference?

Weighing the container with the substance before transfer and then weighing it again after transfer; the difference gives the mass transferred.

How is the mass of substance transferred calculated when weighing by difference?

Mass before transfer − mass after transfer.

Why is weighing by difference more accurate than assuming all the solid was transferred?

It measures the actual mass transferred, including any substance left in the weighing container.

What is the first step in preparing a standard solution?

Place the required solid in a weighing bottle and weigh it accurately.

What should be done after accurately weighing the substance?

Transfer as much as possible to a clean beaker and reweigh the weighing bottle.

Why is the weighing bottle reweighed after transferring the substance?

To calculate the exact mass transferred by difference.

What is added to the solid in the beaker?

About 100 cm³ of deionised water.

What should be done after adding deionised water to the solid?

Stir until all of the solid has dissolved.

Why must all of the solid dissolve before transferring it to the volumetric flask?

So all of the measured substance is present in the solution.

Why is the stirring rod rinsed into the beaker?

To transfer any solution remaining on the rod and prevent loss of solute.

How is the solution transferred from the beaker to the volumetric flask?

Using a funnel.

Why is the beaker rinsed several times with deionised water after transferring the solution?

To ensure all traces of the solute are transferred into the volumetric flask.

What is quantitative transfer?

Transferring all of the measured substance from one container to another without losing any.

Why are the washings from the beaker transferred into the volumetric flask?

To ensure quantitative transfer of the solute.

What is done after all the solution and washings have been transferred to the volumetric flask?

Add deionised water up to the graduation mark.

Where should the bottom of the meniscus be when filling a volumetric flask?

Exactly level with the graduation mark.

At what position should the meniscus be viewed?

At eye level.

Why should the meniscus be viewed at eye level?

To avoid parallax error.

Why should deionised water be added carefully near the graduation mark?

To avoid adding too much water and exceeding the required volume.

What should be done after the volumetric flask has been filled to the graduation mark?

Stopper it and invert it several times.

Why is the volumetric flask inverted several times?

To thoroughly mix the solution and make its concentration uniform throughout.

What happens if some rinsings are discarded instead of transferred to the volumetric flask?

Some solute is lost, so the actual concentration is lower than the calculated concentration.

What happens if water is added above the graduation mark?

The volume is too large, so the actual concentration is lower than the calculated concentration.

What happens to concentration if the same number of moles is dissolved in a larger volume?

The concentration decreases.

What happens to concentration if some of the measured solute is lost during transfer?

The concentration decreases.

How is the concentration of a prepared standard solution calculated?

Calculate the mass transferred, convert the mass to moles using n = m/M, then calculate concentration using c = n/V.

What equation is used to calculate concentration from moles and volume?

c = n/V.

What are the units of concentration normally used for standard solutions?

mol dm⁻³.

How do you calculate moles from mass and molar mass?

n = m/M.

Why is an accurate balance used when preparing a standard solution?

To minimise uncertainty in the measured mass.

Why should deionised or distilled water be used to prepare a standard solution?

It avoids introducing dissolved ions or impurities that could interfere with the solution.

Why is it important to transfer all of the solute into the volumetric flask?

The calculated concentration assumes that the entire measured amount of solute is present in the final solution.

What determines the concentration of a standard solution?

The accurately known number of moles of solute and the accurately known final volume of solution.