Q: A solution of $$MgCl_2$$ has a molarity of 4.0 at 20°C and a density of 1.10 g/mL. What is the weight percent of $$MgCl_2 in $$the solution? (Molar mass of $$MgCl_2 = 95.21$ g/mol)

$$\frac{4.0 \times 95.21}{1.10 \times 1000} \times 100$$

Q: A solution is prepared by dissolving 20 g of solute in 80 g of solvent. What is the weight percent of the solute?

$$\frac{20}{20+80} \times 100 = 20\%$$

Question 1

Which of the following best describes the mole concept in chemical engineering calculations?

Accepted Answer

A mole is the amount of substance that contains Avogadro's number of particles.

Question 2

Given 620 kg of dinitrogen pentoxide ($$N_2O_5$$), how many gram-moles of $$N_2O_5$$ are present? (Molecular weight of $$N_2O_5 = 108 g/$$mol)

Accepted Answer

$$\frac{620,000}{108}$$ gram-moles

Question 3

Which equation correctly expresses the relationship for calculating molarity ($$M) of a $$solution?

Accepted Answer

$$M = \frac{n_{solute}}{V_{soln} (L)}$$

Question 4

A solution of $$MgCl_2$$ has a molarity of 4.0 at 20°C and a density of 1.10 g/mL. What is the weight percent of $$MgCl_2 in $$the solution? (Molar mass of $$MgCl_2 = 95.21$ g/mol)

Accepted Answer

$$\frac{4.0 	imes 95.21}{1.10 	imes 1000} 	imes 100$$

Question 5

Which of the following is the correct formula for specific gravity (S.G.)?

Accepted Answer

$$S.G. = \frac{\rho_{substance}}{\rho_{water}}$$

Question 6

An oil has a specific gravity of 0.81. What is its gravity in degrees API? (Use $$^\circ API = \frac{141.5}{S.G.} - 131.5)$$

Accepted Answer

$$^\circ API = \frac{141.5}{0.81} - 131.5$$

Question 7

In a chemical reaction, which statement best defines the limiting reactant?

Accepted Answer

The reactant that is completely consumed first in the reaction.

Question 8

Given the reaction $$C_6H_{14} + O_2$ \rightarrow H_2O + CO_2$, if 120 g of butane is burned with 600 g of O_2$, which is the limiting reactant? (Molar mass: C_6H$$_{14} = 86$$ g/mol, O_2$ = 32 g/$$mol)

Accepted Answer

$$C_6H_{14}$$

Question 9

Which formula correctly expresses the percent excess reactant?

Accepted Answer

$$\%ER = \frac{Supplied\ ER - Theoretical\ ER}{Theoretical\ ER} 	imes 100$$

Question 10

A solution contains 0.129 wt% $$Mg^{2+}$$ and has a specific gravity of 1.03. What is the concentration of $$Mg^{2+} in $$ppm?

Accepted Answer

$$0.129 	imes 10^4$ ppm

Question 11

Which of the following is NOT a process variable in chemical engineering calculations?

Accepted Answer

Color

Question 12

If the degree of completion for a reaction is 75%, what does this mean?

Accepted Answer

75% of the limiting reactant has been converted to product.

Question 13

Which equation is used to calculate density ($$\rho$$)?

Accepted Answer

$$\rho = \frac{m}{V}$$

Question 14

A solution is prepared by dissolving 20 g of solute in 80 g of solvent. What is the weight percent of the solute?

Accepted Answer

$$\frac{20}{20+80} 	imes 100 = 20\%$$

Question 15

Which of the following best describes the application of stoichiometry in chemical engineering?

Accepted Answer

It is used to interpret quantitative data and apply the laws of conservation of mass and energy.

Question 16

In the Le Blanc process, if only the first reaction is 75% complete and all other reactants are in excess, what is the maximum possible yield of soda ash ($$Na_2CO_3) $$from 4 tonnes of salt ($$NaCl$$)?

Accepted Answer

75% of the theoretical yield based on 4 tonnes of $$NaCl$$

Question 17

Which of the following is the correct formula for molality ($$m$$)?

Accepted Answer

$$m = \frac{moles_{solute}}{mass_{solvent} (kg)}$$

Introduction to Chemical Engineering Calculations: Stoichiometry, Process Variables, and Concentration Expressions

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