Hints for HW4 - second try
Q3. Seawater: The average concentration of chloride ion in seawater is 19400 mg of chloride ion per kg of seawater. What is the molarity of the chloride ion if the density of the seawater is 1.025 g/mL. HINT: Convert mass of seawater to L using density before finding M.
The heart of the question is # of Molarity of stuff.
1) # Molarity = ( # mole of stuff ) / (# liter of solution)
2) You are not given # of liter, but rather 1 kilogram, as the base. So, assume 1 kilogram of solution
3) Convert 1 kilogram of solution to # liter of solution, using the density of seawater. Use dimensional analysis.
4) In that 1 kilogra of solution, the question gives you the mass of stuff. Convert that mass of stuff in # of mole of stuff.
5) Finish up with the definition of Molarity.
Q4c. Barium Chloride: If 55.8 mL of BaCl2 solution is needed to precipitate all the sulfate in a 534 mg sample of Na2SO4 (forming BaSO4), what is the molarity of the solution?
Hint: This is a metathesis reaction, specifically, a precipitation reaction.
1) It is essential to use either a balanced chemical equation
BaCl2(aq) + Na2SO4(aq) ---> BaSO4(s) + 2 NaCl(aq)
____ mol _____ mol
OR
a balanced net ionic equation
Ba2+(aq) + SO4(2-)(aq) ---> BaSO4(s)
____ mol ____ mol
And, I like to set up "_____ mol" under the important units and then fill in the blanks!
Q7c. Mixtures: Indicate the concentration of each ion present in the solution formed by mixing the following. (Assume that the volumes are additive.)
(c) 3.50 g of NaCl in 51.3 mL of 0.514 M CaCl2 solution
Let's think backwards.
1) To get the # Molarity of stuff after mixture, you know the last calculation is
# Molarity = ( # mole of stuff) / (# of liter of total solution )
2) Then, # of liter of total solution = # liter of first solution + # liter of second solution
3) The, # mole of stuff = # mole of stuff in first solution + # mole of stuff in second solution
4a) # mole of stuff in first solution = ( # liter of first solution ) (# molarity of first solution ) ( any stoichiometric conversion factors).
4b) Example of any stoichiometric conversion factors. Let's use CaCl2, then
(stoichiometric conversion factor) = (2 mole Cl- ) / ( 1 mole CaCl2 )
Q8. Precipitation of Fe(2+) ions: What mass of NaOH is needed to precipitate the Fe2+ ions from 70.0 mL of 0.200 M Fe(NO3)2 solution?
1) It essential to use the balanced chemical equation:
Fe(NO3)2 (aq) + 2 NaOH (aq) ---> 2 NaNO3 (aq) + Fe(OH)2 (s)
_______ mol _____ mol
OR the balanced NET ionic chemical equation. Here, I'll use the NET
Fe+2 (aq) + 2 OH- (aq) ---> Fe(OH)2 (s)
_______ mol _____ mol
And I like to put some notes underneath the balanced chemical equation, as shown above.
2) So, fill in the blank for ____ mol of Fe2+ ions.
# mol of Fe2+ ions = (# L of Fe2+ ions) ( # molarity of Fe(NO3)2 ( 1 mole Fe2+ / 1 mole Fe(NO3)2 )
3) # mol of OH- ions = # mol of Fe2+ ions x ( 2 mole OH- ) / (1 mole Fe2+ )
4) And, then convert # mol of OH- ions to # mol of NaOH, and then to # g of NaOH
