A chemist mixes 200.0mL of 0.800 M Fe(NO3)3 and 300.0 mL of 0.750 M K2Cr04 and enough of either 0.400 M KOH or 0.400 M Pb(no3)2 to end up with only two spectator ions in solution.
a. How much precipitate will be formed
b. How many mL of KOH or Pb(No3)2 must be used
c. What are the concentrations of spectator ions in solution
To solve this problem, we need to determine which ionic compounds will form a precipitate by carrying out a double displacement reaction. The remaining ions will be the spectator ions. Let's go through the steps:
Step 1: Write out the balanced chemical equation for the reaction
Fe(NO3)3 + 3K2CrO4 -> Fe2(CrO4)3 + 6KNO3
From the balanced equation, we can see that Fe(NO3)3 and K2CrO4 will react to form Fe2(CrO4)3, which is insoluble and will precipitate. KNO3 will remain in solution as spectator ions.
Step 2: Determine the limiting reactant
To find the limiting reactant, we need to compare the number of moles of each reactant.
Moles of Fe(NO3)3 = volume (L) x molarity (mol/L) = 0.2 L x 0.8 mol/L = 0.16 mol
Moles of K2CrO4 = volume (L) x molarity (mol/L) = 0.3 L x 0.75 mol/L = 0.225 mol
Since Fe(NO3)3 has fewer moles than K2CrO4, it is the limiting reactant.
Step 3: Calculate the moles of precipitate formed
From the balanced equation, we see that the mole ratio between Fe(NO3)3 and Fe2(CrO4)3 is 1:1. Therefore, 0.16 mol of Fe(NO3)3 will produce 0.16 mol of Fe2(CrO4)3.
Step 4: Calculate the mass of precipitate formed
To calculate the mass of Fe2(CrO4)3 precipitate formed, we need to know the molar mass of Fe2(CrO4)3, which is 415.78 g/mol.
Mass of precipitate = moles of precipitate x molar mass of Fe2(CrO4)3
= 0.16 mol x 415.78 g/mol = 66.5 g
Therefore, 66.5 g of precipitate will be formed.
Next, let's move on to part (b) and determine how many mL of KOH or Pb(NO3)2 must be used.
Step 5: Identify the spectator ions
The spectator ions are the remaining ions after the precipitation reaction. In this case, the spectator ions are K+ and NO3- because they are present in both reactants and do not participate in the precipitation reaction.
Step 6: Determine which compound to use
Since we want only two spectator ions in solution, we need to choose either KOH or Pb(NO3)2 to react with the remaining ions.
If we choose KOH, the balanced chemical equation is:
KOH + Fe(NO3)3 -> KNO3 + Fe(OH)3
If we choose Pb(NO3)2, the balanced chemical equation is:
Pb(NO3)2 + Fe(NO3)3 -> 2Pb(NO3) + Fe(NO3)2
Step 7: Calculate the volume of KOH or Pb(NO3)2 required
Using the balanced equation, we can determine the mole ratio between the chosen compound and Fe(NO3)3. From the balanced equations above, we see that the mole ratios are 1:1 for KOH, and 1:1 for Pb(NO3)2.
Let's consider KOH:
Moles of KOH required = moles of Fe(NO3)3 = 0.16 mol
Now, in order to convert moles of KOH to volume, we need to know the molarity of the solution.
Molarity of KOH = 0.4 mol/L
Volume of KOH required = (moles of KOH required) / (molarity of KOH)
= 0.16 mol / 0.4 mol/L = 0.4 L or 400 mL (rounded to 3 significant figures)
Therefore, 400 mL of KOH solution is required.