Intro Chem II questions

Question

I'm preparing for an exam and trying to figure out how to do these questions. Please make show your work, epically formula's!

1. For the reaction C2H6-> 2CH3 rate=k[C2H6]

If k=5.50*10^-4s-1 and [C2H6]=0.0200M calculate the rate of reaction after 1 hour.

2. Given 4PH3-> P4 6H2 rate=k[PH3]

If the rate constant is 0.0278s-1 calculate the percent of the orginal PH3 which has reacted after 76.0s

thanks!

Answer 1

1. The pH of 0.050M aqueous solution of calcium hydroxide

Ca(OH)2 --> Ca+2 & 2OH-

Ksp = [Ca+2] [OH-]^2

6.5e-6 = [0.05] [OH-]^2

OH- = 0.0114

pOH = 1.94

pH = 14 - pOH = your answer : 12.06

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2. For the reaction C2H6-> 2CH3(g) rate=k[C2H6]

if K= 5.50*10^-4 and [C2H6]intial=0.0200M calculate the rate reaction after 1 hour.

for first order reactions:

log [At] / [Ao] = - kt/ 2.303

log [At] / [Ao] = - (5.50E-4)(3600 seconds)/ 2.303

log [At] / [Ao] = - 0.860

1O^x both sides

[At] / [Ao] = 0.138

[At] = 0.138 [Ao]

[At] = 0.138 [0.0200] = 2.76e-3 Molar after 1 hour

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rate=k[PH3]

rate= 5.50*10^-4 [2.76e-3]

your answer: rate = 1.52 e-6 mol/litre-sec

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3. given 4PH3(g)-> P4(g) 6H2(g) rate=k[PH3]

if the rate constant is 0.0278s-1, calculate the percent of the orginal PH3 which reacted after 76.0 s

log [At] / [Ao] = - kt/ 2.303

log [At] / [Ao] = - (0.0278s-1)(76 seconds)/ 2.303

log [At] / [Ao] = - 0.917

1O^x both sides

[At] / [Ao] = 0.1209

% = [At] / [Ao] times 100 = 12.09 % is left

your answer(3sigfigs): 12.1%