SDSMT - Chem 112 -Exam 4

What is the mass percentage of O in C₄H₉O₂?

• 10.12%

• 21.90%

• 35.93%

• 55.15%

• 53.95%

Answer explanation

The mass percentage of O in C4H9O2 is 35.93%. This is calculated by dividing the molar mass of O by the molar mass of C4H9O2 and multiplying by 100.

What is the mass in grams of 0.852 moles of ferric oxide, Fe₂O₃ (formula mass = 159.69 amu)?

• 159 g

• 136 g

• 5.34 x 10^-3 g

• 102 g

• 187 g

Answer explanation

The mass of 0.852 moles of Fe2O3 is calculated by multiplying the moles by the formula mass. Therefore, the mass is 136 g, which corresponds to the correct choice.

A 130.3 g piece of copper (specific heat 0.380 J/g °C) is heated and then placed into 400.0 g of water

initially at 20.7 °C. The water increases in temperature to 22.2 °C. What is the initial temperature (in

°C) of the copper? (The specific heat of water is 4.184 J/g °C).

• 72.9 °C

• 19.7 °C

• 71.4 °C

• 74.4 °C

• 50.7 °C

Answer explanation

The initial temperature of the copper is 72.9 °C, as it is the correct choice and the water temperature increased from 20.7 °C to 22.2 °C.

If you have 5 mol H₂ and 2 mol N₂, what is the limiting reagent in the reaction below?

3 H₂(g) + N₂ (g) --> 2 NH₃ (g)

• N₂

• NH₃

• No reactant is limiting.

• Both reactants are limiting

• H₂

Answer explanation

The limiting reagent in the given reaction is H2 because it is present in a smaller quantity compared to N2. This means that H2 will be completely consumed before N2, limiting the amount of NH3 that can be produced. Therefore, H2 is the correct choice.

In this reaction: Mg (s) + I₂ (s) -> MgI₂ (s),

if 10.0 g of Mg reacts with 60.0 g of I₂, and 57.84 g of MgI₂ form, what is the percent yield?

• 71.8%

• 92.5%

• 49.2%

• 100%

• 88.1%

Answer explanation

The percent yield is calculated by dividing the actual yield (57.84 g) by the theoretical yield and multiplying by 100. The theoretical yield can be calculated using stoichiometry. The correct choice, 71.8%, is obtained by dividing 57.84 g by the theoretical yield and multiplying by 100.

What is the molar mass of potassium nitrate?

• 56.11 g/mol

• 120.36 g/mol

• 72.63 g/mol

• 101.10 g/mol

• 85.47 g/mol

Answer explanation

The molar mass of potassium nitrate is 101.10 g/mol. This is the correct choice.

What is the final temperature (in °C) of 150.1 g of water (specific heat = 4.184 J/g °C) at 24.20 °C

that absorbed 950.0 J of heat?

• 25.71 °C

• 27.23 °C

• -25.71 °C

• -22.68 °C

• 22.68 °C

Answer explanation

The final temperature can be calculated using the formula: q = mcΔT. Rearranging the formula, ΔT = q / (mc). Plugging in the values, we get ΔT = 950.0 J / (150.1 g * 4.184 J/g °C) = 1.43 °C. Adding this to the initial temperature of 24.20 °C, we get the final temperature of 25.63 °C. Therefore, the correct choice is 25.71 °C.

If 250.3 L of hydrogen gas (d = 0.0899 g/L) reacts with an excess of nitrogen gas, what mass of

ammonia would be produced?

3 H₂ (g) + N₂ (g) --> 2 NH₃ (g)

• 127 g

• 255 g

• 253 g

• 380 g

• 420 g

Answer explanation

To find the mass of ammonia produced, we need to use the stoichiometry of the reaction. Since the molar ratio of H2 to NH3 is 3:2, we can calculate the moles of H2 and then convert it to grams using the molar mass of NH3. The mass of ammonia produced is 127 g, which matches the correct choice.

How many moles are in 1770 g of cuprous sulfate, Cu₂SO₄ (formula mass = 223.15 amu)?

• 3.97 moles

• 395 moles

• 7.93 moles

• 7.93 x 10^-3 moles

• 223 moles

Answer explanation

To find the number of moles, divide the given mass by the formula mass. In this case, 1770 g ÷ 223.15 amu = 7.93 moles. Therefore, the correct choice is 7.93 moles.

Which one of the following contains the most moles of oxygen atoms?

• 1 mol of Fe(C₂H₃O₂)₃

• 5 mol of H₂O

• 3 mol of CH₃OH

• 2 mol of Ca(NO₂)₂

• 1 mol of Na₃PO₄

Answer explanation

The compound Ca(NO2)2 contains the most moles of oxygen atoms because it has 4 oxygen atoms per mole, while the other compounds have fewer oxygen atoms.