344 ❯ STEP 5. Build Your Test-Taking ConfidenceUse the following information to answer questions
6–9.
Many metal salts crystallize from solution in the
form of a hydrate. If the formula of the anhy-
drous salt is AaXx, then the generic formula of the
hydrated form would be AaXx•xH 2 O. A student
conducts an experiment to determine the for-
mula of a metal oxide by collecting the following
data:
Mass of crucible 53.120 g
Mass of crucible plus hydrated salt 58.677 g
Mass of crucible plus anhydrous salt 57.857 g
Molar mass of anhydrous salt 208.232 g mol-^1
The hydrated salt is finely powdered to ensure
complete water loss to produce the anhydrous salt.- How many grams of water were in the hydrate?
(A) 5.557 g
(B) 4.737 g
(C) 0.987 g
(D) 0.820 g - What is the approximate percent water in the
hydrated salt?
(A) 14%
(B) 86%
(C) 50%
(D) 28% - The student did not have time to finish the experi-
ment during the lab period and was forced to store
the anhydrous salt and crucible in his lab drawer
until the next day. At the beginning of the next
lab period the student weighed the anhydrous salt
and beaker. After calculating the percent water in
the sample, the student found that the percentage
was lower than predicted. Assuming all weighing
were done correctly, what might be the cause of
the lower than expected percentage?
(A) The sample dried further overnight.
(B) The student performed one or more weigh-
ings before the crucible had cooled to room
temperature.
(C) The sample absorbed water from the air
overnight.
(D) The sample decomposed to another salt.
9. In another experiment on a different metal
hydrate a student found that the salt was 62.9%
water. In this case, the molar mass of the anhy-
drous salt was 106 g mol-^1. Which of the follow-
ing general formulas gives the correct value of x?
(A) AaXx•5H 2 O
(B) AaXx•10H 2 O
(C) AaXx•2H 2 O
(D) AaXx•6H 2 O
10. For the following reaction, 2 NO(g) + O 2 (g) →
2 NO 2 (g), the rate law is: Rate = k[NO]^2 [O 2 ]. In
one experiment, the rate of appearance of NO 2
was determined to be 0.0138 M s-^1 when [NO] =
0.0125 M and [O 2 ] = 0.0125 M. What was the
value of the rate constant?
(A) 7.1 × 103 M^2 s-^1
(B) 7.1 × 103 M s-^1
(C) 7.1 × 103 M-^2 s-^1
(D) 7.1 × 103
Use the following information to answer questions
11–12.EXPERIMENTINITIAL
[H+] (M)INITIAL
[CN 2 H 4 O]
(M)INITIAL
RATE OF
DISAPPEARANCE
OF CN 2 H 4 O
(M s−1)
1 0.100 0.200 8.6 × 10 -^5
2 0.200 0.100 4.3 × 10 -^5
3 0.200 0.200 8.6 × 10 -^5- The table above gives the initial concentrations
and rate for three experiments involving the
decomposition of urea, CN 2 H 4 O. The reac-
tion is H+(aq) + 2 H 2 O(l) + CN 2 H 4 O(aq) →
2 NH 4 +(aq) + HCO 3 - (aq). What is the rate law
for this reaction?
(A) Rate = k[CN 2 H 4 O]
(B) Rate = k[CN 2 H 4 O]^2 [H+]^2
(C) Rate = k[H+]
(D) Rate = k[CN 2 H 4 O]^2 [H+]
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