18. Metallic copper can be obtained from the mineral chalcocite (Cu:S). What is

Question

18. Metallic copper can be obtained from the mineral chalcocite (Cu:S). What is the mass percent of copper in chalcocite? a. 20.15% b. 33.54% c. 79.85% d. e. 66.46% 57.47% 19. If there are 0.505 g of NaCI left in a beaker that originally contained 75.0 mL of saltwater, what must have been the concentration of the original solution? a. 0.0673 M b. 0.00647 M c. 0.673 M d. 0.115 M e. 0.0115 M Which one of the following statements regarding a weak acid is not correct? a. 20. b. c. d. e. A weak acid ionizes only to a small extent in water. All acids are weak at low concentrations. A weak acid ionizes in water to produce hydronium ions. A weak acid neutralizes bases. Acetic acid is an example of a weak acid

Explanation / Answer

Solution:

18. The reasons for high copper content (66.6% atomic ratio and nearly 80% by weight) in chalcolite (Cu2S) is ease of which copper can be separated from sulfur.

Ans: Option (d) 66.46%

19. Molar mass of NaCl = 58.44 g/mol

     Wt. of NaCl = 0.505 g

     Volume = 75.0 mL = 0.075 lt

     Molarity = moles / volume

                 = (0.505/58.44)/0.075

                 = 0.1152 M

Ans: Option (d) 0.115 M

20. Ans: Option (b) all acids are weak at low concentrations.

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