Every calculation you make about a solution starts with a concentration somebody wrote on a label. Today you are the somebody. By the end of the period you will have made a solution whose concentration you can state to three significant figures and defend, and you will have made a second one from the first without weighing anything at all.
The chemistry here is nothing. The technique is everything, and the technique is the reason the number on the label means anything.
What you are trying to find out
Two questions that look like housekeeping and are not.
How accurately can you actually make a solution? You will be asked for 0.100 mol/L. Every step — the weighing, the transfer, the dissolving, the making up to the mark — carries an uncertainty, and they do not all push the same way. Working out where the largest one lives, and how big it is as a percentage, is the real content of the day.
Does diluting change the amount of solute? Take a measured volume of your solution, add water, and you have a second solution at a different concentration. Something changed and something did not. Say precisely which, and then use that sentence to derive the relationship you need rather than looking it up.
There is a third, smaller question that finishes the period: can you show there is what you say there is in the bottle? A concentration is a quantitative claim, and it rests on a qualitative one — that the solute is the substance you think it is.
What you have to work with
- Sodium chloride, solid, and a balance reading to 0.01 g or better.
- 250.0 mL volumetric flask with a stopper, and a 100.0 mL volumetric flask. A funnel. A wash bottle of distilled water.
- A beaker of roughly 100 mL, and a stirring rod.
- Pipette and pipette bulb or pump, and a small measuring cylinder.
- For the confirmation test: silver nitrate solution, dispensed by me, one drop at a time.
Your design decisions and calculations, done before the lab and checked before you weigh anything:
- The mass of sodium chloride needed for 250.0 mL of 0.100 mol/L. Show , then , with the molar mass to two decimal places.
- How many significant figures that mass is entitled to, and what the balance’s resolution does to it as a percentage of the mass you are weighing. That percentage propagates straight into your concentration and it is probably not the biggest error of the day — work out what is.
- The volume of your stock solution needed to make 100.0 mL of 0.0500 mol/L, derived from your own sentence about what dilution does and does not change.
- Which piece of glassware you will measure that volume with, and why. A measuring cylinder, a graduated pipette, and a volumetric pipette are three different answers with three different uncertainties.
Thin glass, a corrosive stain, and one rule with no exceptions
- Never pipette by mouth. Not for the solution, not for the distilled water, not “just this once because it is only salt water”. A bulb or a pump, every single time. This rule exists because people who broke it are why we have the rule.
- Volumetric glassware is thin-walled and expensive. Never heat it, never dry it in an oven, never use it to stir, and set it down on a mat rather than on a hard bench edge. A cracked flask is a cut hand.
- Silver nitrate is corrosive and it stains skin black. The stain takes days to grow out and it does not wash off. I dispense it, one drop, into a spot plate or a small test tube — it never comes to your bench in a bottle. Gloves for that step, and hands washed after. Silver waste goes into its own labelled container.
- Eye protection on throughout, including during the weighing and including while you wait.
- Today’s dilution is of a salt solution and releases no appreciable heat. When you dilute an acid — next lab, and in the titration — the rule is add acid to water, never water to acid. The heat of dilution spreads through a large volume of water when the water is already in the flask; poured the other way it concentrates in a few drops that can boil and throw acid back out at your face.
- Never return unused solution or unused solid to the stock container. Take what you need; the remainder goes to waste.
- Waft, do not sniff. Nothing is tasted, including sodium chloride solution, which is not table salt once it has been near lab glassware.
- Report any breakage, splash, or spill the moment it happens.
The transfer sequence, and why every step is in it
Run this out loud with your partner before you start, and tick it as you go. Skipping any one of these is how a solution ends up at the wrong concentration while looking perfect.
- Weigh the solid on a watch glass or weighing boat, and record the mass actually weighed, not the mass you wanted
- Dissolve it in a beaker, in distinctly less water than the final volume — never in the volumetric flask itself
- Stir until no solid remains anywhere, including under the rod
- Let the solution come back to room temperature before it goes near the mark
- Transfer to the volumetric flask through a funnel
- Rinse the beaker, the rod, and the funnel with distilled water three times, and put every rinsing into the flask
- Add distilled water to just below the mark
- Finish drop by drop with the wash bottle, eye level with the graduation, until the bottom of the meniscus sits on the line
- Stopper and invert at least ten times, turning it right over each time — a solution that is not mixed is not a solution
- Label the flask: substance, concentration, your name, today
Two of those steps carry the whole logic. You dissolve in less than the final volume because a solid takes up room once it is dissolved; if you dissolved it in exactly 250.0 mL of water you would end up with more than 250.0 mL of solution. And you make up to the mark in the volumetric flask, never in a beaker, because beaker graduations are decorative — they are moulded, not calibrated, and are commonly out by several percent.
The prediction you write first
Before the balance is touched:
- The mass you will weigh, in grams, to the right number of figures.
- The volume of stock you will take for the dilution.
- Your estimate of the largest single source of uncertainty in the final concentration, named and expressed as a percentage.
- Which direction you expect your errors to push the concentration overall — high or low — and why. Commit to a direction before you find out.
- What you expect to see when a drop of silver nitrate meets your solution, and what you would conclude if you saw nothing.
What to collect
| Measurement | Value | Unit | Instrument and its resolution |
|---|---|---|---|
| Target concentration | 0.100 | mol/L | — |
| Calculated mass required | g | — | |
| Mass actually weighed | g | ||
| Volume of the volumetric flask | 250.0 | mL | |
| Concentration actually prepared | mol/L | — | |
| Volume of stock taken for dilution | mL | ||
| Final volume after dilution | 100.0 | mL | |
| Concentration of the diluted solution | mol/L | — |
The row that matters most is concentration actually prepared, which is calculated from the mass you really weighed and not from the mass you meant to weigh. Those two numbers are different and only one of them is a measurement.
| Confirmation test | What you added | What you observed | What it supports |
|---|---|---|---|
| Your stock solution | one drop of silver nitrate | ||
| Distilled water (control) | one drop of silver nitrate |
The control is not optional. A positive test on distilled water would mean your “distilled” water is not, and you would want to know that before you trusted anything else on the page.
What to bring to the consolidation discussion
- Your prepared concentration, to three significant figures, with the arithmetic shown.
- A short uncertainty budget: each step, the uncertainty it contributes as a percentage, and which one dominates. This is the piece of work I will read most carefully.
- Your dilution calculation, derived from the sentence about what stays constant rather than quoted.
- The confirmation test result, with the control.
- One sentence on what you would change to halve your largest uncertainty, and what it would cost in time or equipment.
What you should not claim
- The label is not the measurement. Writing “0.100 mol/L” on the flask does not make it so. What you actually prepared is fixed by the mass you weighed and the flask you used, and it is almost certainly not 0.100 exactly. Report what you made.
- Most of the ways this goes wrong push the concentration down, and it is worth noticing that they all share a mechanism: solute that never made it into the flask. Crystals left on the weighing boat, solution left in the beaker or on the stirring rod, undissolved solid at the bottom, and water added past the mark all lower the concentration. Once you are past the mark it cannot be corrected by removing liquid, because the liquid you remove contains solute.
- One thing pushes the concentration up, and it is easy to miss. Making the solution up to the mark while it is still warm from dissolving means the liquid contracts as it cools, so the flask ends up holding slightly less than 250.0 mL of solution with all of the solute still in it. This is why “let it come to room temperature” is a step and not a courtesy.
- Parallax is a real error and its direction depends on where your eye is. Rather than argue about which way, remove it: get your eye level with the graduation, and read the bottom of the meniscus.
- A white precipitate with silver nitrate is consistent with chloride, not proof of it. Other ions give a white precipitate with silver too. This is a screening test, and calling it an identification is claiming more than one drop can carry.
- You have not measured your concentration. You have constructed it. Everything on this page rests on the assumptions that the solid was pure and dry and that the flask is calibrated. Measuring a concentration is a different job, and it is the next investigation: Titrating an Acid.
Curriculum connection
E2.3
prepare solutions of a given concentration by dissolving a solid solute in a solvent or by diluting a concentrated solution [PR]
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E2.4
conduct an investigation to analyse qualitative and quantitative properties of solutions (e.g., perform a qualitative analysis of ions in a solution) [PR, AI]
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A1.2
select appropriate instruments (e.g., a balance, glassware, titration instruments) and materials (e.g., molecular model kits, solutions), and identify appropriate methods, techniques, and procedures, for each inquiry
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