1 · Trace one error
The silver chloride is weighed while still wet. Which mass changes first? Then which way does the % silver go?
AP Chemistry · Topic 1.4 · Composition of mixtures
Run three real analyses of mixtures, then break them. Every lab error changes a mass you weighed; follow it to the moles and then to the percent, and see which way the result moves.
| # | Analysis | Errors | Product or gas (g) | Sample (g) | Result (%) | True (%) |
|---|
Record adds the current run. Copy puts the table on the clipboard for a spreadsheet.
The silver chloride is weighed while still wet. Which mass changes first? Then which way does the % silver go?
Spill 5% of the dissolved alloy and record. Then clear it and make the sample 1.5 times bigger. Which one is really an error?
Try dissolved CO₂, water spray and MgCO₃ in the shell, one at a time. Which push the % CaCO₃ up, and which down?
Divide the silver by the AgCl instead of the alloy. Change the alloy's true composition: does that answer ever change?
About the model. Each analysis follows one element into something you can weigh: every carbon atom in the shell's carbonate leaves as CO₂, every silver atom ends in AgCl, every barium atom ends in BaSO₄. So the moles of that element in the product equal the moles in the sample, with no balanced equation needed (the equation route is Topic 4.5). The standard samples are the Topic 1.4 deck's example data: an eggshell of 1.215 g that loses 0.508 g of CO₂ (95.1% CaCO₃), a 0.500 g sterling-silver alloy that gives 0.614 g of AgCl, and a 5.20 g BaCl₂ + KCl mixture that gives 3.15 g of BaSO₄.
Each error changes one measured mass by the amount you set, and everything after it is calculated the student's way. The arrows compare each number with an error-free run of the same sample. The rest of a real eggshell is protein and other matter that releases no gas; in this model MgCO₃, when you add it, replaces some of that matter, so the true % CaCO₃ stays the same. Mass percent of a solution is not part of this topic and isn't used here.
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