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Even though Ca(OH) 2 is an inexpensive base, its limited solubility restricts its use. What is the pH of a saturated solution of Ca(OH) 2 ?
What mass of NaCN must be added to 1 L of 0.010 M Mg(NO 3 ) 2 in order to produce the first trace of Mg(OH) 2 ?
5.4 10 −3 g
Magnesium hydroxide and magnesium citrate function as mild laxatives when they reach the small intestine. Why do magnesium hydroxide and magnesium citrate, two very different substances, have the same effect in your small intestine. (Hint: The contents of the small intestine are basic.)
The following question is taken from a Chemistry Advanced Placement Examination and is used with the permission of the Educational Testing Service.
Solve the following problem:
In a saturated solution of MgF 2 at 18 °C, the concentration of Mg 2+ is 1.21 10 –3 M . The equilibrium is represented by the preceding equation.
(a) Write the expression for the solubility-product constant, K sp , and calculate its value at 18 °C.
(b) Calculate the equilibrium concentration of Mg 2+ in 1.000 L of saturated MgF 2 solution at 18 °C to which 0.100 mol of solid KF has been added. The KF dissolves completely. Assume the volume change is negligible.
(c) Predict whether a precipitate of MgF 2 will form when 100.0 mL of a 3.00 10 –3 - M solution of Mg(NO 3 ) 2 is mixed with 200.0 mL of a 2.00 10 –3 - M solution of NaF at 18 °C. Show the calculations to support your prediction.
(d) At 27 °C the concentration of Mg 2+ in a saturated solution of MgF 2 is 1.17 10 –3 M . Is the dissolving of MgF 2 in water an endothermic or an exothermic process? Give an explanation to support your conclusion.
(a)
K
sp = [Mg
2+ ][F
– ]
2 = (1.21
10
–3 )(2
1.21
10
–3 )
2 = 7.09
10
–9 ; (b) 7.09
10
–7
M
(c) Determine the concentration of Mg
2+ and F
– that will be present in the final volume. Compare the value of the ion product [Mg
2+ ][F
– ]
2 with
K
sp . If this value is larger than
K
sp , precipitation will occur.
0.1000 L
3.00
10
–3
M Mg(NO
3 )
2 = 0.3000 L
M Mg(NO
3 )
2
M Mg(NO
3 )
2 = 1.00
10
–3
M
0.2000 L
2.00
10
–3
M NaF = 0.3000 L
M NaF
M NaF = 1.33
10
–3
M
ion product = (1.00
10
–3 )(1.33
10
–3 )
2 = 1.77
10
–9
This value is smaller than
K
sp , so no precipitation will occur.
(d) MgF
2 is less soluble at 27 °C than at 18 °C. Because added heat acts like an added reagent, when it appears on the product side, the Le Châtelier’s principle states that the equilibrium will shift to the reactants’ side to counter the stress. Consequently, less reagent will dissolve. This situation is found in our case. Therefore, the reaction is exothermic.
Which of the following compounds, when dissolved in a 0.01- M solution of HClO 4 , has a solubility greater than in pure water: CuCl, CaCO 3 , MnS, PbBr 2 , CaF 2 ? Explain your answer.
Which of the following compounds, when dissolved in a 0.01- M solution of HClO 4 , has a solubility greater than in pure water: AgBr, BaF 2 , Ca 3 (PO 4 ) 3 , ZnS, PbI 2 ? Explain your answer.
BaF 2 , Ca 3 (PO 4 ) 2 , ZnS; each is a salt of a weak acid, and the from perchloric acid reduces the equilibrium concentration of the anion, thereby increasing the concentration of the cations
What is the effect on the amount of solid Mg(OH) 2 that dissolves and the concentrations of Mg 2+ and OH – when each of the following are added to a mixture of solid Mg(OH) 2 and water at equilibrium?
(a) MgCl 2
(b) KOH
(c) HClO 4
(d) NaNO 3
(e) Mg(OH) 2
What is the effect on the amount of CaHPO 4 that dissolves and the concentrations of Ca 2+ and when each of the following are added to a mixture of solid CaHPO 4 and water at equilibrium?
(a) CaCl 2
(b) HCl
(c) KClO 4
(d) NaOH
(e) CaHPO 4
Effect on amount of solid CaHPO 4 , [Ca 2+ ], [OH – ]: (a) increase, increase, decrease; (b) decrease, increase, decrease; (c) no effect, no effect, no effect; (d) decrease, increase, decrease; (e) increase, no effect, no effect
Identify all chemical species present in an aqueous solution of Ca 3 (PO 4 ) 2 and list these species in decreasing order of their concentrations. (Hint: Remember that the ion is a weak base.)
A volume of 50 mL of 1.8 M NH 3 is mixed with an equal volume of a solution containing 0.95 g of MgCl 2 . What mass of NH 4 Cl must be added to the resulting solution to prevent the precipitation of Mg(OH) 2 ?
9.2 g
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