Which solution containing an equal number of moles of each of the substances is

Question

Which solution containing an equal number of moles of each of the substances is a buffer? A) H_2SO_3 and H_2SO_4 B) KBr and HBr C) H_2CO_3 and NaHCO_3 D) Na_2HPO_4 and H_3PO_4 E) More than one of the solutions above is a buffer. How many moles of gas are contained in a scuba diver's 12.6-L tank that is filled with 243 atm of air at 25 degree C? A) 1490 moles B) 2.10 times 10^4 moles C) 125 moles D) 1760 moles What is the [H_3O^+] of an acid rain sample that has a pH of 3.45? A) 2.8 times 10^3 M B) 3.5 times 10^-4 M C) 3. 4 M D) 11 M Three of the four phase changes below are exothermic. Which phase change is NOT exothermic? A) sublimation B) freezing C) condensation D) deposition

Explanation / Answer

1)

acidic buffer is formed by weak acid and salt of weak acid

basic buffer is formed by weak base and salt of weak base

A is not a buffer since both are acid

B is not a buffer since HBr is strong acid

C is buffer (H2CO3 is weak acid and NaHCO3 is the salt)

D is not a buffer since salt of H3PO4 will be NaH2PO4 and not Na2HPO4

Answer: C

2)

Given:

P = 243 atm

V = 12.6 L

T= 25.0 oC = (25.0+273) K = 298.00 K

use:

P * V = n*R*T

243 atm * 12.6 L = n * 0.0821 atm.L/mol.K * 298.0 K

n = 125.15 mol

Answer: C) 125 mol

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