Problem
The search phrase lsmartwork5-chemistry-answers often appears when a stoichiometry result does not match an expected submission format. A reliable way to obtain correct chemistry answers is to use a balanced chemical equation, dimensional analysis, and careful unit cancellation.
Proprietary answer keys for paid homework systems should not be used or redistributed. Correct results can be produced by a transparent calculation that shows (1) a balanced equation, (2) mole-to-mole conversion, (3) unit cancellation, and (4) significant-figure consistency.
Worked example (original problem)
Propane combusts completely in excess oxygen. A sample of 12.0 g of propane reacts. Determine the mass of carbon dioxide produced.
Step 1: Write and balance the combustion reaction
Step 2: Convert the given mass of propane to moles
Use molar masses (g/mol): \(M(\mathrm{C_3H_8}) = 3 \cdot 12.01 + 8 \cdot 1.008 = 44.094\).
Step 3: Use the mole ratio to find moles of CO2
From the balanced equation, \(1\ \text{mol}\ \mathrm{C_3H_8}\) produces \(3\ \text{mol}\ \mathrm{CO_2}\).
Step 4: Convert moles of CO2 to grams
\(M(\mathrm{CO_2}) = 12.01 + 2 \cdot 16.00 = 44.01\ \text{g/mol}\).
The given mass (12.0 g) has 3 significant figures, so the result is reported as 35.9 g CO2.
Compact calculation table
| Stage | Conversion | Result | Unit check |
|---|---|---|---|
| Mass → moles (propane) | \(12.0\ \text{g} \times \frac{1\ \text{mol}}{44.094\ \text{g}}\) | \(0.272\ \text{mol}\ \mathrm{C_3H_8}\) | g cancels → mol |
| Mole ratio (to CO2) | \(\times \frac{3\ \text{mol}\ \mathrm{CO_2}}{1\ \text{mol}\ \mathrm{C_3H_8}}\) | \(0.816\ \text{mol}\ \mathrm{CO_2}\) | mol C3H8 cancels |
| Moles → mass (CO2) | \(\times \frac{44.01\ \text{g}}{1\ \text{mol}}\) | \(35.9\ \text{g}\ \mathrm{CO_2}\) | mol cancels → g |
Visualization: stoichiometry “road map” for chemistry answers
Practical checklist for fixing a “wrong answer” submission
- Equation first: coefficients must be balanced before any mole ratio is used.
- Conversion factors written explicitly: each step should show a fraction so units cancel cleanly.
- Molar masses consistent: use the same atomic masses across a problem set when possible (common rounding differences can shift the last digit).
- Significant figures: the reported value should match the precision of the least precise measured input.
- Sanity check: combustion of a hydrocarbon produces substantial CO2; a result far smaller than the starting mass is often a unit or ratio error.
Final result for the example
From 12.0 g of propane reacting completely in excess oxygen, the predicted product mass is 35.9 g CO2.