Two clear solutions go into a beaker and a white solid appears. The equation says exactly how much of that solid there should be. Your job is to get it out of the beaker, dry it, weigh it, and find out how close the equation’s number and the balance’s number actually come.
They will not agree. The question worth a whole period is by how much, in which direction, and why.
What you are trying to find out
Every quantitative calculation you have done so far assumed the reaction goes to completion and the product ends up in your hand. Neither assumption survives contact with a filter funnel.
Percentage yield is the number that measures the gap:
The theoretical yield comes from the balanced equation and the limiting reagent. The actual yield comes from the balance. So this investigation is really three investigations stacked:
- Can you decide, in advance and on paper, which reagent limits the reaction — and then arrange for it to be the one you chose?
- Can you recover a solid quantitatively, which is a skill and not a formality?
- Can you account for the gap between the two numbers with mechanisms rather than apologies?
A yield above 100% is not a failure and is not impossible. It is a message about your procedure, and by the end of the page you should be able to read it.
What you have to work with
- Sodium carbonate solution, 0.500 mol/L
- Calcium chloride solution, 0.500 mol/L
- Beakers, stirring rod, wash bottle of distilled water
- Filter funnel and filter paper, or a BĂĽchner funnel if the room has one
- Watch glass, drying oven or drying rack, desiccator if available
- Balance, with its resolution written on it
The reaction, which you should balance and check before you calculate anything:
Your design decisions, written down and checked before you dispense anything:
- Which reagent you make limiting, and by how much you make the other one exceed it. Not “roughly more”. A stated percentage excess, with a reason. Too little excess and you are not sure the limiting reagent was fully consumed; too much and you are wasting reagent and making the filtrate harder to deal with.
- Your scale. Choose volumes so the mass of precipitate is at least a couple of hundred times the balance’s resolution. Show the calculation that justifies your choice.
- Your filtration and washing plan. How many washes, with what, and how you will know the washing is finished. Washing removes soluble salts clinging to the crystals — and every wash also risks losing precipitate.
- Your drying plan, with a stopping rule. As in the last investigation, the rule is constant mass: dry, cool, mass, repeat until two successive masses agree to within the balance’s resolution.
- What you mass, and when. The filter paper must be massed dry and before use. Decide now, because you cannot go back for it.
An alkaline solution, a hot oven, and glassware that lies
- Sodium carbonate solution is alkaline and irritating, to eyes especially. Eye protection stays on from the first pour to the last of the cleanup, and cleanup is when most splashes happen.
- Rinse any splash on skin under running water and tell me. An alkaline solution does not sting the way an acid does, so the feeling of “it is probably fine” is exactly the feeling to distrust. Slippery skin means rinse longer.
- The drying oven and anything in it are hot, and hot glass looks identical to cold glass. Tongs or heat-resistant gloves for everything, and a watch glass out of the oven goes on a mat, never on the bench and never straight on the balance.
- Never place a warm object on the balance. Rising air off it drags the reading downward and you will conclude you lost product that you did not lose.
- Never pipette by mouth. Bulb or pump, for every liquid.
- Waft, do not sniff. Nothing is tasted. Your product is not chalk for drawing on the pavement and it is not food.
- Never return unused solution to a stock bottle. Filtrate and solid waste go to the labelled containers by the routes on your station card.
- Report every spill and every splash the moment it happens, however small it seems.
The prediction you write first
Before any solution is measured out:
- Which reagent is limiting, shown with a calculation, not asserted.
- The theoretical yield in grams, to the correct number of significant figures. Molar masses to two decimal places.
- The percentage yield you expect, as a number with a range around it, and — this is the part that matters — which side of 100% you expect to land on, and why.
- The single largest source of loss or gain you expect, named, with a guess at how many grams it accounts for.
Prediction 3 is the one people want to leave vague. Do not. “We expect somewhere between 80% and 95%, because filtration losses dominate” is a claim that can be wrong, and being wrong about it is the most useful outcome available today.
What to collect
| Measurement | Value | Unit | Resolution |
|---|---|---|---|
| Concentration of sodium carbonate solution | mol/L | ||
| Volume of sodium carbonate solution | mL | ||
| Concentration of calcium chloride solution | mol/L | ||
| Volume of calcium chloride solution | mL | ||
| Mass of dry filter paper | g | ||
| Mass of dry watch glass | g | ||
| Mass after first drying | g | ||
| Mass after second drying | g | ||
| Mass after third drying | g | ||
| Mass of dry precipitate | g |
And the mass account, which is where the thinking shows:
| Quantity | Value (g) | How you got it |
|---|---|---|
| Theoretical yield | From the limiting reagent | |
| Actual yield | From the balance, at constant mass | |
| Difference | Signed — say which is larger | |
| Percentage yield |
Record observations alongside: how quickly the precipitate formed, whether the supernatant liquid was clear or still cloudy when you filtered, how long the filtration took, whether any solid passed through. A cloudy filtrate is a measured loss and it belongs in the account.1
What to bring to the consolidation discussion
- Your mass account, complete, with the percentage yield to a defensible number of significant figures.
- Your identification of the limiting reagent, with the calculation.
- A ranked list of the mechanisms that could account for your gap, each one with the direction it pushes the yield and a rough size in grams. Ranked, not listed.
- Whether the total of your named mechanisms actually accounts for the gap you observed. If it does not, say so — an unexplained residue is a real result.
- The class spread of percentage yields. Look at whether the values scatter around one number or pile up on one side of it.
What you should not claim
- A yield below 100% is not “the reaction did not finish”. That is one possibility among several, and it is usually not the main one. In a precipitation like this, product left on the beaker wall, on the stirring rod, and in the pores of the filter paper is almost always larger than any incompleteness in the reaction itself.
- A yield above 100% is not extra product and it is not a mistake to hide. Mass appeared that was not calcium carbonate. The three usual routes, all of which push the number up: water that had not finished leaving the solid; soluble sodium chloride left behind because the precipitate was not washed enough; and filter paper that was massed dry, then took up moisture from the air before the final weighing. Name which one you think it was and say why.
- “We lost some” is not a source of error. The report needs the route and the direction. Fine particles passed through the filter paper and left with the filtrate, which lowers the actual yield is a sentence with content. Some was lost is not.
- Your percentage yield is a property of your afternoon, not of the reaction. Repeat it with a BĂĽchner funnel and it changes. Quote it as what it is: the fraction you recovered under your procedure.
- Do not report more figures than the weakest measurement allows. If the theoretical yield rests on a volume measured in a graduated cylinder, the third figure of your percentage yield is decoration.
- Two decimal places on the balance is not two decimal places on the answer. Work out what your resolution does to the final percentage before you write it down. A precipitate of 2.31 g on a balance reading to 0.01 g already carries about half a percent of uncertainty, before any of the mechanisms above are counted.
Curriculum connection
D2.7
conduct an inquiry to determine the actual yield, theoretical yield, and percentage yield of the products of a chemical reaction (e.g., a chemical reaction between steel wool and copper(II) sulfate solution), assess the effectiveness of the procedure, and suggest sources of experimental error [PR, AI]
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D2.6
solve problems related to quantities in chemical reactions by performing calculations involving percentage yield and limiting reagents [AI]
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A1.8
synthesize, analyse, interpret, and evaluate qualitative and quantitative data; solve problems involving quantitative data; determine whether the evidence supports or refutes the initial prediction or hypothesis and whether it is consistent with scientific theory; identify sources of bias and error; and suggest improvements to the inquiry to reduce the likelihood of error
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Footnotes
-
Weighing a product to find out how much of something there was is called gravimetric analysis, and before instrumental methods existed it was how quantitative chemistry was done — ore assays, water hardness, the composition of new compounds. Its accuracy lived and died on exactly the two steps you are about to find fiddly: washing the precipitate properly, and drying it to a mass that stops changing. The technique is unglamorous and it built the periodic table’s numbers. ↩