Question
Which of the following numerical expressions gives the number of moles in 5.0g of CaO?
A \(5.0g\times 56g/mol\)
B\(\frac{5.0g}{56g/mol}\)
C\(\frac{56g/mol}{5.0g}\)
D\(\frac{1}{5.0g}\times \frac{1}{50g/mol}\)
▶️Answer/Explanation
Ans:B
Multiplying the mass of the sample by the molar mass of CaO gives an irrelevant value and unit. Dividing the mass of the sample by the molar mass of CaO would give the number of moles of CaO in the sample.
Question
In a lab, a student is given a 21g sample of pure Cu metal. Which of the following pieces of information is most useful for determining the number of Cu atoms in the sample? Assume that the pressure and temperature in the lab are 1.0atm and 25°C?
- The molar mass of Cu.
- The density of Cu at 25°C.
- The volume of the Cu sample.
- The ratio of the two main isotopes found in pure Cu.
▶️Answer/Explanation
Ans-A
The density could be used with the given mass to determine the volume of the Cu sample, but this would not be helpful for determining the number of Cu atoms in the sample. However, by dividing the mass of the sample by the molar mass of Cu, the number of moles of Cu in the sample can be calculated. A simple multiplication of that number with Avogadro’s number will provide the desired information.
Question
1.0 mol sample of which of the following compounds has the greatest mass?
A \(NO\)
B. \(NO_{2}\)
C \(N_{2}O\)
D \(N_{2}O_{5}\)
▶️Answer/Explanation
Ans:D
The mass of 1.0mol of NO is less than the mass of 1.0mol of N2O5 because it has fewer N and O atoms and thus a smaller molar mass.
Question
\(C_3H_8(g) + 4 Cl_2(g) → C_3H_4Cl(g) + 4 HCl(g)\)
A 6.0 mol sample of\( C_{3}H_8(g)\) and a 20. mol sample of \(Cl_{2}\)(g) are placed in a previously evacuated vessel, where they react according to the equation above. After one of the reactants has been totally consumed, how many moles of HCl(g) have been produced?
(A) 4.0 mo
(B) 8.0 mol
(C) 20. mol
(D) 24 mol
▶️Answer/Explanation
Ans:C
Question
\(CO_2(g) + 2 LiOH(s) → Li_2CO3(aq) + H_2O\)(l)
In a one-person spacecraft, an astronaut exhales 880 g of \(CO_2\)(g) (molar mass 44 g/mol) per day. To prevent the buildup of \(CO_{2}\)(g) in the spacecraft, a device containing LiOH(s) is used to remove the \(CO_2\)(g), as represented by the equation above. What mass of LiOH(s) (molar mass 24 g/mol) is needed to react with all of the \(CO_2\)(g) produced by an astronaut in one day?
(A) 40. g
(B) 240 g
(C) 480 g
(D) 960 g
▶️Answer/Explanation
Ans:D