5 Steps to a 5 AP Chemistry 2019

AP Chemistry Practice Exam 2 ❮ 367

Question 4

(a) The Ideal Gas Equation is one way of calculating the number of moles, n, of a gas.

Moles = n =

PV

RT

= (^) −−
(1.00atm)(1.00L)
(0.0821Latm molK^11 )(273K)
= 0.0446 mol HCN (three significant figures)
As an alternative, if you recognize the 273 K and 1.00 atm is STP, you can use the molar volume of a
gas at STP to calculate the number of moles.
Moles = n =











(1.00L) 

1 mol

22.4 L

= 0.0446 mol HCN (three significant figures)

This answer is worth 1 point.

(b) Again it is possible to use the Ideal Gas Equation to solve for the volume, V, of a gas.

Volume = V =

nRT

P

=



































(1.00 g Ca(CN))  −−

1 mol Ca(CN)

92.1 g Ca(CN)

2 mol HCN

1 mol Ca(CN)

(0.0821 L atm mol K)(298 K)

(1.00 atm)

2

2

22

11

=

0.531 L (three significant figures)

This answer is worth 1 point. Unlike part (a) you cannot use the molar volume of a gas because the

conditions are not STP.

(c) Based upon its structure, HCN is polar and, therefore, must be soluble in water. For this reason, some

will not be in the gas phase but in the solution, which means the volume of the gas will be less than

expected.

This answer, including the explanation, is worth 1 point.

(d)

H C N

There must be a single bond between the hydrogen atom and the carbon atom and a triple bond

between the carbon atom and the nitrogen atom. The lone pair on the nitrogen atom can be anywhere

as long as it is obvious that it is only associated with the nitrogen atom. Both the carbon and the nitro-

gen atoms have an octet of electrons. This answer is worth 1 point.

Total your points for each part. There are 4 possible points. Subtract 1 point if all reported answers

did not have the correct number of significant figures.

21-Moore_PE02_p341-370.indd 367 31/05/18 1:54 pm