Preparing a Standard Solution: Grade 12 Formal Assessment
A standard solution is one whose concentration you know accurately. Making one is a Grade 12 formal assessment, and it is the step that decides whether the titration after it means anything.
This page covers what a standard solution is, what a primary standard has to be, the calculation, the method step by step, and the one idea the whole task exists to teach: your solution will not come out at exactly 0,1 mol·dm-3, and that is not a mistake.
What a standard solution is
A standard solution is a solution whose concentration is known accurately.
That is the definition, and the word doing the work is accurately. Not approximately, not "about 0,1". A number you can write down and rely on.
You need one because a titration compares an unknown against a known. If the known is not actually known, the whole titration is arithmetic performed on a guess.
Why you cannot just weigh out any chemical
To make a solution of known concentration you weigh a solid and dissolve it in a known volume. Which means the solid has to be exactly what the label says it is.
A solid that meets that standard is called a primary standard.
| A primary standard must be | Why |
|---|---|
| Very pure | Impurities mean part of the mass you weighed is not the substance |
| Stable in air | It must not react with oxygen or carbon dioxide while it sits on the balance |
| Not hygroscopic | A solid that pulls water out of the air gets heavier while you weigh it |
| Of known, fixed composition | Including any water of crystallisation, which has to be in the formula |
| Reasonably high in molar mass | A bigger mass to weigh means the weighing error matters less |
The ones you will meet
| Primary standard | Formula | Makes a standard | M (g·mol-1) |
|---|---|---|---|
| Oxalic acid dihydrate, also called ethanedioic acid dihydrate | (COOH)2·2H2O | Acid | 126 |
| Succinic acid, butanedioic acid | (CH2COOH)2 | Acid | 118 |
| Anhydrous sodium carbonate | Na2CO3 | Alkali | 106 |
| Sodium hydrogen carbonate | NaHCO3 | Alkali | 84 |
And the one that is not a primary standard, which is the point
Sodium hydroxide cannot be a primary standard, and this is worth understanding rather than memorising.
- It absorbs water from the air. Pellets left out get visibly wet
- It reacts with carbon dioxide in the air to form sodium carbonate
So a bottle labelled 0,1 mol·dm-3 sodium hydroxide is not reliably 0,1 mol·dm-3, however carefully it was made.
That is exactly why the next practical exists. You make an accurate acid, then use it to find out what the sodium hydroxide really is. That is what standardising means.
The water of crystallisation trap
The formula is (COOH)2·2H2O, and the two waters are part of the mass you weigh out.
| What you use | M | What happens |
|---|---|---|
| (COOH)2·2H2O, the dihydrate | 126 g·mol-1 | Correct |
| (COOH)2 on its own, the anhydrous form | 90 g·mol-1 | You weigh out 2,25 g instead of 3,15 g and your solution is 29 % too dilute |
This is the single most common error in the whole task. Every crystal of the solid you are holding carries two water molecules with it, and they contribute 36 of the 126 g·mol-1.
Where this fits in the curriculum
| Subject | Physical Sciences |
|---|---|
| Grade | 12 |
| Term | 2 |
| Topic | Chemical change, acids and bases |
| Status | FORMAL ASSESSMENT |
| Marks | 40 |
There are two formal assessments in this chapter, back to back, and most schools only run the second one.
- This one: prepare the standard solution
- The next one: use it to standardise a sodium hydroxide solution
They are one continuous piece of work. Doing the titration with a solution somebody else made removes the part the assessment is actually about.
The calculation
Two steps, and they run in this order every time.
n = c × V, then m = n × M
Worked example: the task itself
Find the mass of oxalic acid dihydrate needed for 250 cm³ of a 0,1 mol·dm-3 solution.
- Convert the volume to dm³ first. 250 cm³ ÷ 1 000 = 0,250 dm³
- n = c × V = 0,1 × 0,250 = 0,025 mol
- M((COOH)2·2H2O) = 24 + 2 + 64 + 36 = 126 g·mol-1
- m = n × M = 0,025 × 126
- m = 3,15 g
Worked example: an alkali, and a different volume
Find the mass of anhydrous sodium carbonate needed for 500 cm³ of a 0,05 mol·dm-3 solution.
- V = 500 ÷ 1 000 = 0,500 dm³
- n = 0,05 × 0,500 = 0,025 mol
- M(Na2CO3) = 46 + 12 + 48 = 106 g·mol-1
- m = 0,025 × 106 = 2,65 g
Same number of moles, different substance, different mass. The moles are what the chemistry cares about. The grams are only how you get them onto a balance.
The idea this whole assessment exists to teach
You will not weigh out 3,15 g. You will weigh out something close to it, and then you work out what you actually made.
Nobody can hit a target mass exactly with a spatula. So the instruction is not "weigh 3,15 g". It is weigh out a mass approximately equal to the mass required, record it accurately, and calculate the concentration from what you actually weighed.
Worked example: working out what you actually made
A learner aiming for 3,15 g records:
| Reading | Mass |
|---|---|
| Watch glass, empty | 24,86 g |
| Watch glass + oxalic acid dihydrate | 28,04 g |
| Mass of solid, by difference | 3,18 g |
- n = m ÷ M = 3,18 ÷ 126 = 0,02524 mol
- c = n ÷ V = 0,02524 ÷ 0,250
- c = 0,10 mol·dm-3, to two decimal places
Two things to notice.
The mass is found by difference, never by trying to zero a balance with a watch glass balanced on it. Weigh the watch glass, weigh it again with the solid on, subtract.
And the answer is your solution's concentration, not the one you were aiming at. Carry it forward. A learner who writes 3,18 g in the table and then uses 0,100 mol·dm-3 in the titration has thrown away the accuracy they just spent a period creating.
The method
Apparatus
| Item | Qty | Note |
|---|---|---|
| Volumetric flask, 250 cm³ | 1 | The one piece that cannot be substituted. A measuring cylinder is not accurate enough |
| Balance reading to 0,01 g | 1 | Two decimals is enough. See below |
| Watch glass | 1 | To weigh the solid on |
| Spatula | 1 | |
| Small funnel | 1 | The neck of a volumetric flask is narrow and unforgiving |
| Wash bottle of distilled water | 1 | Distilled, not tap. Tap water carries dissolved ions |
| Beaker, 100 cm³ | 1 | For dissolving before transfer |
| Stirring rod | 1 | |
| Oxalic acid dihydrate | ~4 g per group | Primary standard grade |
Steps
- Calculate the mass you need before you touch anything. 3,15 g for 250 cm³ of 0,1 mol·dm-3.
- Weigh the empty watch glass and write the reading down.
- Add solid until you are close to the target, then weigh again and write that down. Do not chase the exact figure. Close is the instruction.
- Tip the solid into a beaker and dissolve it in a small amount of distilled water, stirring. Dissolve it here, not in the flask.
- Pour the solution through the funnel into the volumetric flask.
- Rinse the beaker, the rod and the funnel into the flask with distilled water from the wash bottle. Two or three times.
- Add distilled water until the flask is about half full, stopper it and swirl until everything is fully dissolved.
- Fill to the graduation mark, dropwise at the end, until the bottom of the meniscus sits on the line at eye level.
- Stopper and invert the flask about ten times, turning it right over each time.
- Calculate the actual concentration from the mass you actually weighed.
Three steps that look optional and are not
| Step | What goes wrong without it |
|---|---|
| Rinsing the beaker into the flask | Solid left behind in the beaker was weighed but never made it into the solution. Your solution is more dilute than your calculation says, and nothing in your working reveals it |
| Dissolving before making up to the mark | Undissolved solid sitting on the bottom means the concentration is wrong now and changes later |
| Inverting to mix | Water added last sits on top. The top of the flask is more dilute than the bottom, and the first pipette you draw is not representative |
Why a volumetric flask and nothing else
A volumetric flask has one graduation mark on a narrow neck, and that is the whole design.
A narrow neck means a large change in height for a small change in volume, so you can judge the mark to a fraction of a millilitre. A 250 cm³ measuring cylinder is wide, and a millimetre of height is several millilitres of liquid.
| Glassware | Typical tolerance at 250 cm³ | Fit for this task |
|---|---|---|
| Volumetric flask, Class A | ±0,15 cm³ | Yes |
| Volumetric flask, Class B | ±0,30 cm³ | Yes, for school work |
| Measuring cylinder | ±2 cm³ or worse | No |
| Beaker graduations | ±5 % or worse | Not measuring apparatus at all |
Class A carries a batch certificate, which is the flask telling you its own tolerance. For an assessment whose entire subject is accuracy, that is the honest choice.
Why the mark is on the neck and not the body
Glass expands. A volumetric flask is calibrated at 20 °C, and putting a warm solution into it puts more liquid below the mark than there should be once it cools.
Let the solution reach room temperature before the final fill. Dissolving oxalic acid does not warm it much, but rinsing with warm water does.
Does a 0,01 g balance give enough accuracy?
Yes, comfortably, and it is worth being able to show why.
Weighing 3,15 g on a balance that reads to 0,01 g gives an uncertainty of about 0,3 %. The flask contributes about 0,06 %. The mass is the bigger of the two and it is still small.
A three-decimal analytical balance is not needed for this. If your school has one, use it. If it has a 0,01 g balance, you are fine, and the task asks for the concentration to two decimal places anyway.
What would ruin it is a 0,1 g balance, which turns 0,3 % into 3 % and makes the whole exercise pointless.
If it does not work
| What happens | What caused it |
|---|---|
| You overshoot the graduation mark | Start again. Do not pour any out. The flask now holds more than 250 cm³ and there is no way to correct it |
| Solid will not dissolve in the flask | You skipped the beaker. Tip it back out into a beaker, dissolve, and start the transfer again |
| The concentration comes out about 29 % low | You used 90 g·mol-1 instead of 126. The water of crystallisation was left out of the molar mass |
| The concentration comes out ten times out | cm³ not converted to dm³. Divide by 1 000 |
| Crystals appear in the flask a day later | Undissolved solid, or the solution was made up warm and has cooled |
| Two groups using the same bottle get different answers | Correct and expected. Each group weighed a different mass. Each has a different, and equally valid, concentration |
| The solid looks damp and clumped | The bottle has been left open. Damp solid weighs partly water, and the concentration will read low |
Safety
- Oxalic acid is harmful if swallowed and irritates skin and eyes. Goggles and gloves, and wash hands afterwards
- It is a fine solid. Do not create dust and do not blow it off the balance pan
- Wipe up spilt solid with a damp cloth rather than brushing it
- The solution is dilute and low hazard, but it is still an acid. Rinse spills with plenty of water
- Never pipette by mouth
How the 40 marks are made up
| Section | Marks |
|---|---|
| Apparatus and chemicals identified | 6 |
| The method in the correct order | 10 |
| Calculating the mass required | 8 |
| Calculating the actual concentration | 10 |
| Accuracy and sources of error | 6 |
No mark allocation is prescribed for this one. The split above is ours, weighted towards the two calculations because that is what the exam tests when the practical itself is over.
Free worksheet and marking memo
Both free, no sign up, straight to the PDF.
- Learner worksheet, 40 marks, with the definitions, both calculations, the method in scrambled order to sequence, and an accuracy question
- Marking memorandum, with full working, the mark breakdown line by line, and a note on the six places learners drop marks
Related practicals
- Titration, Grade 12. The other half of this assessment. Uses the solution you just made
- Percentage yield, Grade 11. The other practical that cannot be done without a balance
- Chemical equilibrium. Also Term 2, Grade 12
Apparatus
Every item for this practical is in stock, which is not something we can say about many of them.
The one that matters is the 250 cm³ volumetric flask. We carry Class A with a batch certificate, Class B, and polypropylene. For a formal assessment about accuracy, buy the Class A. The certificate is the point, and a school buys one flask per group once and keeps it for a decade.
You will also need a balance reading to 0,01 g. It is the same balance the percentage yield practical needs, and between the two of them it earns its place in a school laboratory.