Chemistry Spring Final Math Review

A student is analyzing a compound that contains 52.2% carbon, 13.0% hydrogen, and 34.8% oxygen by mass. Determine the empirical formula of the compound.

• C4H10O2

• C2H6O

• C4H11O2

• C3H9O

Answer explanation

To find the empirical formula, convert the percentages to moles: C (52.2g/12.01), H (13.0g/1.008), O (34.8g/16.00). This gives approximately 4.36 C, 12.87 H, and 2.18 O. Dividing by the smallest (2.18) yields C2H6O, the correct empirical formula.

Using the empirical formula (C2H6O) and the given molar mass, determine the molecular formula. The molar mass of the compound is 92 g/mol.

• C8H20O4

• C4H12O2

• C8H24O4

• C5H16O2

Answer explanation

To find the molecular formula, divide the molar mass (92 g/mol) by the empirical formula mass (46 g/mol for C2H6O). This gives 2, so multiply the subscripts in the empirical formula by 2, resulting in C4H12O2.

Using the molecular formula C4H12O2, how many molecules are present in a 138 grams sample of this compound?

• 6.02 x 10^23 molecules

• 1.38 x 10^24 molecules

• 1.23 x 10^23 molecules

Answer explanation

To find the number of molecules in 138 grams of C4H12O2, first calculate its molar mass (C: 12.01, H: 1.01, O: 16.00) = 88.16 g/mol. Then, use the formula: molecules = (mass/molar mass) x Avogadro's number = (138 g / 88.16 g/mol) x 6.02 x 10^23 = 9.03 x 10^23 molecules.

Tags

NGSS.HS-PS1-7

What is the percent composition of iron in the compound Fe2O3?

• 69.94%

• 30.06%

• 55.95%

• 34.97%

Answer explanation

To find the percent composition of iron in Fe2O3, calculate the molar mass: Fe (55.85 g/mol) x 2 + O (16.00 g/mol) x 3 = 159.70 g/mol. The percent composition of iron is (2 x 55.85 / 159.70) x 100 = 69.94%.

When 8.00 grams of hydrogen gas (H2) react with excess oxygen gas (O2), how many grams of water (H2O) should theoretically be produced?

• 18.0 grams

• 36.0 grams

• 71.5 grams

• 144.0 grams

Answer explanation

To find the mass of water produced, use the reaction: 2 H2 + O2 -> 2 H2O. 8.00 g H2 (4 moles) produces 4 moles of H2O. The molar mass of H2O is 18 g/mol, so 4 moles x 18 g/mol = 72 g. The closest answer is 71.5 grams.

Tags

NGSS.HS-PS1-7

If 70.0 grams of water are actually collected, what is the percent yield of the reaction?

• 92.3%

• 97.9%

• 100%

• 94.6%

Answer explanation

To find the percent yield, divide the actual yield (70.0 g) by the theoretical yield and multiply by 100. If the theoretical yield is 71.4 g, then (70.0 g / 71.4 g) x 100 = 97.9%. Thus, the correct answer is 97.9%.

• 0.625 mol NaCl

• 625 mol NaCl

• 36.5 g NaCl

• 0.04 mol NaCl

Answer explanation

To find moles of NaCl, use the formula: mol = M × L. Here, M = 5.00 M and L = 0.125 L. Thus, mol = 5.00 × 0.125 = 0.625 mol. Therefore, the correct answer is 0.625 mol NaCl.

What is the molarity of a 250. mL H2SO4 solution that was made from a 20.0 mL of a 10.0 M stock solution?

• 0.800 M

• 1.25 M

• 125 M

• 500. M

Answer explanation

Using M1V1 = M2V2, where M1 = 10.0 M, V1 = 20.0 mL, and V2 = 250. mL, we find M2 = (M1V1)/V2 = (10.0 M * 20.0 mL) / 250. mL = 0.800 M. Thus, the molarity is 0.800 M.

All of the following containers have one mole of gas and have the same volume and pressure. Which container has the molecules with the slowest average kinetic energy?

• Container 1

• Container 2

• Container 3

• Container 4

Answer explanation

The average kinetic energy of gas molecules is directly proportional to temperature. Since all containers have the same volume and pressure, the one with the lowest temperature will have the slowest average kinetic energy. Thus, Container 4 is correct.

Tags

NGSS.HS-PS3-2

Determine which of the following scenarios BEST matches with the correct corresponding Law of Thermodynamics. A lamp is plugged into the wall, and electrical energy is used to make light energy.

• Zeroth Law

• First Law

• Second Law

• Third Law

Answer explanation

The scenario describes energy conversion, where electrical energy is transformed into light energy. This aligns with the First Law of Thermodynamics, which states that energy cannot be created or destroyed, only transformed.