Question

Suppose 0.250 g of cobalt(II)
chloride hexahydrate is dissolved in 50.0 mL of 4 M HCl, and heated
to 80^{o}C in a
cuvette having a 1.00-cm path length. At this temperature, the
percent transmittance of the solution at 690 nm is measured to be
48.0%.

What is the equilibrium
concentration of CoCl_{4}^{2-} in the solution? (Hint: use Beer's
Law)

a. 2.10 × 10^{-2} M

b. 8.25 × 10^{-2} M

c. 1.81 × 10^{3} M

d. 5.52 × 10^{-4} M

What is the equilibrium concentration of
Co(H_{2}O)_{6}^{2+}?

a. 5.52 × 10^{-4} M

b. 2.16 × 10^{-2} M

c. 2.05 × 10^{-2} M

d. 2.11 × 10^{-2} M

Answer #1

**1st part**

Absorbance, A = 2 - log_{10} %T = 2 - log_{10}
48 = 0.32

Again **A=ebc**

**e** is the molar absorbtivity with units of L
mol^{-1} cm^{-1
b is the path length of the sample
and}^{c is the concentration of the
compound in solution, expressed in mol L-1}

Therefore c = A/eb = 0.32/ e x 1 = 0.32/ e = 0.32/ 557.2 =
**5.54 x 10-4 M**

[The reported molar absorptivity value for [CoCl4]2- at around 700 nm is 577.2 M-1cm-1. ]

**So d is the answer**

**2nd part:**

Conc. of Co(H2O)6 2+ = (conc. of cobalt(II) chloride
hexahydrate) - (conc. of [CoCl_{4}]^{2-})

= (0.250/ 237.93)/(50/1000) - 5.54 x 10-4 M = **2.05 x
10 ^{-2} M so C is the answer**

You titrate 50.0 mL of 0.100 M benzoic acid (C6H5COOH) with
0.250 M KOH. What is the concentration of C6H5COO- at the
equivalence point? The answer is: [C6H5COO-] = 7.14 x 10-2 M., but
I'm not sure why?
What is the pH of the final solution at the equivalence point?
Ka of C6H5COOH = 6.3 x 10-5.

1.What volume of a 1.50 M NaOH solution should be added to 50.0
mL of 1.20 M acetic acid (CH3CO2H;
Ka = 1.76×10-5) to obtain a buffer with pH =
5.450?
a) 33.3 mL
b) 6.70 mL
c) 16.4 mL
d) 67.0 mL
e) 42.1 mL
2. Find the concentration of hydronium ion,
H3O+, in a 0.200 M solution of sodium
hypochlorite, NaOCl.
Ka(HOCl) = 3.0×10-8.
a) 7.8×10-5 M
b) 1.3×10-10 M
c) 1.0×10-7 M
d) 3.9×10-11 M
e) 2.6×10-4...

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