The mole and stoichiometry: flashcards, quiz and summary

Flashcards

1. 1Why do chemists count atoms in moles instead of one by one?

   Atoms are far too small to count individually, so chemists use the mole, which is 6.022 × 10²³ particles, to work with measurable amounts.

2. 2How is the molar mass of a compound such as H2O found?

   Add the molar masses of all the atoms in the formula. For H2O, 2(1.01) + 16.00 = 18.02 g/mol.

3. 3What are the steps to convert grams of a substance to grams of another substance in a reaction?

   Convert grams of the first substance to moles using its molar mass, use the mole ratio from the balanced equation to find moles of the second substance, then convert those moles to grams using its molar mass.

4. 4Why must an equation be balanced before doing stoichiometry?

   Conservation of mass requires the same number of atoms of each element on both sides, and the coefficients in the balanced equation provide the mole ratios used in calculations.

5. 5Why is it wrong to change subscripts when balancing an equation?

   A subscript defines the substance itself. Changing it creates a different substance, so only coefficients may be changed.

6. 6How do you identify the limiting reactant in a reaction?

   Convert each reactant to moles, calculate how much product each could make using the mole ratio, and the reactant that produces less product is limiting.

7. 7Why can the limiting reactant have more moles than the other reactant?

   The limiting reactant is determined by the mole ratio in the balanced equation, not by the raw mole amounts. A reactant with more moles can still run out first if the equation requires more of the other reactant per mole.

8. 8Why is the theoretical yield calculated from the limiting reactant and not the excess reactant?

   The limiting reactant runs out first, so it caps how much product can form. The excess reactant is left over and cannot make more product than the limiting reactant allows.

9. 9What does a percent yield below 100% usually indicate?

   Some product was lost, because of side reactions, incomplete reactions or spills, so the actual yield was less than the theoretical yield.

10. 10What could cause a percent yield above 100%?

    A product that is wet or impure, since extra mass from water or other contaminants makes the collected product heavier than the theoretical yield.

Practice quiz

Pick an answer to check it.

1. 1What is the molar mass of H2O using H = 1.01 g/mol and O = 16.00 g/mol?

   1. 18.02 g/mol
   2. 16.00 g/mol
   3. 20.02 g/mol
   4. 17.01 g/mol
   Show the answer

   A. 18.02 g/mol Two hydrogen atoms contribute 2(1.01) = 2.02, and one oxygen contributes 16.00, giving a total of 18.02 g/mol. The other choices come from leaving out an atom or using the wrong number of hydrogen atoms.

2. 2Which step should come first when balancing an equation?

   1. Balance elements that appear in only one substance on each side
   2. Change the subscripts so the atoms match
   3. Reduce coefficients to the smallest whole numbers
   4. Balance H and O atoms
   Show the answer

   A. Balance elements that appear in only one substance on each side Balance elements found in only one substance per side first, and to save H, O and lone elements for last. Changing subscripts is never allowed, and reducing coefficients is the final step.

3. 3In the reaction 2 H2 + O2 → 2 H2O, 2.00 mol of H2 and 1.500 mol of O2 are present. Which reactant is limiting?

   1. O2, because it has fewer moles
   2. Neither, because the moles are close
   3. H2, because it makes less water
   4. H2O, because it is the product
   Show the answer

   C. H2, because it makes less water H2 can make 2.00 mol of water while O2 can make 3.00 mol. The reactant that makes less product is limiting, so H2 is the limiting reactant even though it has more moles.

4. 4The theoretical yield of water in a reaction is 36.04 g and the actual yield is 32.4 g. What is the percent yield?

   1. 90.0%
   2. 111.2%
   3. 10.1%
   4. 89.9%
   Show the answer

   D. 89.9% The percent yield is 32.4 ÷ 36.04 × 100%, which equals 89.9%. The value 90.0% is a rounding trap, and 111.2% comes from dividing in the wrong direction.

5. 5A student multiplies 36.04 g of H2O by 6.022 × 10²³ to get molecules. What is the main error?

   1. No error, because particles are found by multiplying grams
   2. The grams must first be divided by molar mass to get moles
   3. The student should use 6.02 × 10²³ instead
   4. The student should use the molar mass of H2 instead
   Show the answer

   B. The grams must first be divided by molar mass to get moles Grams must be converted to moles first by dividing by molar mass. Multiplying grams directly by Avogadro's number gives the wrong number of particles and units that do not cancel.

6. 6Why is the number of moles of a limiting reactant important when calculating yield?

   1. It shows the percent yield directly
   2. It sets the maximum amount of product that can form
   3. It is always equal to the mass of the excess reactant
   4. It determines the molar mass of the product
   Show the answer

   B. It sets the maximum amount of product that can form The limiting reactant runs out first, so its moles, combined with the mole ratio, cap the product. The other options confuse yield with molar mass or percent yield, which are separate calculations.

Test traps

What teachers like to test on this, and the mistakes students make.

• Students often assume the reactant with fewer moles is limiting, but the limiting reactant must be found by comparing the product each reactant could make using the mole ratio.
• Students use coefficients as mass ratios, forgetting to convert grams to moles first before applying the mole ratio from the balanced equation.
• Students change subscripts to balance an equation, which creates a different substance instead of balancing the original one.
• Students calculate theoretical yield from the excess reactant, when it must come from the limiting reactant, which caps the product.
• Students divide actual yield by theoretical yield but forget to multiply by 100%, or reverse the division so the percent yield comes out above 100%.
• Students confuse molar mass, which is grams per mole, with the number of particles per mole, which is always 6.022 × 10²³.

Summary

• One mole is 6.022 × 10²³ particles (Avogadro's number), which lets chemists count atoms, molecules, ions or formula units in measurable amounts.
• Molar mass is the mass of one mole of a substance in g/mol; for elements it matches the average atomic mass on the periodic table, and for compounds you add the atomic masses of all atoms in the formula.
• Converting between grams, moles and particles uses division by molar mass and multiplication by Avogadro's number, with dimensional analysis to make units cancel.
• A balanced equation obeys conservation of mass, so each element has the same number of atoms on both sides, and only coefficients may be changed, never subscripts.
• Coefficients in a balanced equation are mole ratios, not mass ratios, so grams must be converted to moles before using the ratio.
• The limiting reactant is the one that runs out first and caps the amount of product; find it by calculating how much product each reactant could make and choosing the smaller result.
• Theoretical yield comes from the limiting reactant, and percent yield equals actual yield divided by theoretical yield times 100%, usually below 100% because of side reactions, incomplete reactions or spills.
• At STP, 1 mole of any gas occupies about 22.4 L, although some books use 1 bar as the standard pressure, which gives about 22.7 L.

Key terms

Mole

A counting unit equal to 6.022 × 10²³ particles, such as atoms, molecules, ions or formula units.

Avogadro's number

The number of particles in one mole, 6.022 × 10²³; since 2019 it is defined exactly as 6.02214076 × 10²³.

Molar mass

The mass of one mole of a substance, measured in g/mol.

Stoichiometry

The use of a balanced chemical equation to predict the amounts of reactants used and products formed.

Conservation of mass

The principle that atoms are neither created nor destroyed in a reaction, so each element has the same number of atoms on both sides of the equation.

Mole ratio

The ratio of moles between two substances in a balanced equation, taken from the coefficients.

Limiting reactant

The reactant that is used up first in a reaction, which determines the maximum amount of product that can form.

Excess reactant

The reactant that is not completely used up when the limiting reactant runs out.

Theoretical yield

The maximum amount of product that can be calculated from the limiting reactant.

Percent yield

The actual yield divided by the theoretical yield, multiplied by 100%.

STP

Standard temperature and pressure, 0 °C and 1 atm, where 1 mole of any gas fills about 22.4 L.

How to study it

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Questions