DAT - Dental Admission General Chemistry: Stoichiometry Questions and Answers

Question 1

In the complete combustion of butane (C4H10), what is the sum of the stoichiometric coefficients of the reactants and products in the balanced chemical equation?

Accepted Answer

33

Answer

The balanced chemical equation for the complete combustion of butane is 2 C4H10 + 13 O2 → 8 CO2 + 10 H2O. To find the sum of the coefficients, you add them together: 2 + 13 + 8 + 10 = 33. It's crucial to balance the equation first, starting with carbons, then hydrogens, and finally oxygens.

Question 2

A compound is found to contain 40.0% carbon, 6.7% hydrogen, and 53.3% oxygen by mass. If the molar mass of the compound is approximately 180 g/mol, what is its molecular formula?

Accepted Answer

C6H12O6

Answer

First, assume a 100g sample, which gives 40.0g C, 6.7g H, and 53.3g O. Convert these masses to moles: C (40.0g / 12.01 g/mol ≈ 3.33 mol), H (6.7g / 1.01 g/mol ≈ 6.63 mol), O (53.3g / 16.00 g/mol ≈ 3.33 mol). Divide by the smallest number of moles (3.33) to get the empirical formula CH2O. The molar mass of the empirical formula (CH2O) is about 30 g/mol. To find the molecular formula, divide the given molar mass by the empirical formula mass (180 g/mol / 30 g/mol = 6). Multiply the subscripts in the empirical formula by this factor (6) to get C6H12O6.

Question 3

Consider the reaction: 2 Al(s) + 6 HCl(aq) → 2 AlCl3(aq) + 3 H2(g). If 10.8 g of aluminum (Al) reacts with 36.5 g of hydrochloric acid (HCl), which of the following statements is correct? (Molar masses: Al = 27.0 g/mol, HCl = 36.5 g/mol)

Accepted Answer

Hydrochloric acid is the limiting reactant, and 0.5 moles of H2 are produced.

Answer

First, convert the mass of each reactant to moles: Moles of Al = 10.8 g / 27.0 g/mol = 0.4 mol. Moles of HCl = 36.5 g / 36.5 g/mol = 1.0 mol. According to the stoichiometry, 2 moles of Al react with 6 moles of HCl, a 1:3 ratio. For 0.4 mol of Al, 1.2 mol of HCl would be needed (0.4 * 3). Since only 1.0 mol of HCl is available, HCl is the limiting reactant. Now, calculate the moles of H2 produced from the limiting reactant: 1.0 mol HCl * (3 mol H2 / 6 mol HCl) = 0.5 mol H2.

Question 4

In a laboratory experiment, the reaction of 32.5 g of zinc with excess hydrochloric acid produced 0.90 g of hydrogen gas. What is the percent yield of hydrogen? The balanced equation is: Zn(s) + 2 HCl(aq) → ZnCl2(aq) + H2(g). (Molar masses: Zn = 65.4 g/mol, H2 = 2.0 g/mol)

Accepted Answer

90.0%

Answer

First, calculate the theoretical yield of H2. Convert the mass of Zn to moles: 32.5 g Zn / 65.4 g/mol ≈ 0.497 mol Zn. According to the 1:1 mole ratio between Zn and H2, 0.497 moles of H2 should be produced. Convert moles of H2 to grams: 0.497 mol H2 * 2.0 g/mol = 0.994 g H2 (theoretical yield). The actual yield is given as 0.90 g. The percent yield is (Actual Yield / Theoretical Yield) * 100 = (0.90 g / 0.994 g) * 100 ≈ 90.5%, which is closest to 90.0%.

Question 5

Which of the following correctly defines the empirical formula of a compound?

Accepted Answer

The formula representing the simplest whole-number ratio of atoms of each element in the compound.

Answer

The empirical formula represents the simplest whole-number ratio of elements within a compound. For example, the molecular formula for glucose is C6H12O6, but its empirical formula is CH2O, as the ratio 6:12:6 can be simplified to 1:2:1.

DAT Practice Test

DAT Practice Test

DAT - Dental Admission General Chemistry: Stoichiometry Questions and Answers