Solution to pre-lab question
K
2
SO
4
(aq) + BaCl
2
(aq)  2KCl(aq) +
Na
2
SO
4
(aq) + BaCl
2
(aq)  2NaCl(aq) +
BaSO
BaSO
4
4
(s)
(s)
Prelab question:
0.100g of solid mixture yields 0.0676g of BaSO
4
. What is the % of each component?
MM BaSO
4
= 233.4g/mole n
BaSO4
= 0.676g (1mole/233.4g) = 2.90 x 10 -4 moles
Because of the 1:1 ratio in both equations:
1.
moles of BaSO
4
= moles of K
2
SO
4
+ moles of Na
2
SO
4
= 2.90 x 10 -4 moles
2.
Mass of sample = mass of K
2
SO
4
+ mass of Na
2
SO
4
= 0.10g
By definition: Mass = moles x Molar mass mass of K
2
SO
4
= (moles of K
2
SO
4
)x 174.3g/mole mass of Na
2
SO
4
= (moles of Na
2
SO
4
) x 142.1g/mole
Substitute these two expressions into equation 2 above gives:
3.
(moles of K
2
SO
4
)x 174.3g/mole+ (moles of Na
2
SO
4
) x 142.1g/mole= 0.10g
Solve equation 1 for moles of moles of Na
2
SO
4
gives:
4.
moles of Na
2
SO
4
= 2.90 x 10 -4 moles - moles of K
2
SO
4
Substitute equation 4 into equation 3.
5.
(moles of K
2
SO
4
)x 174.3g/mole+ (2.90 x 10 -4 moles - moles of K
2
SO
4
) x
142.1g/mole= 0.10g
To simplify let moles of K
2
SO
4
) be x and leave out the units.
6.
174.3x + 142.1(2.90 x 10 -4 – x)= 0.10
Solve this algebra equation for x gives 1.86 x 10 -4 = x = moles of K
2
SO
4
Mass of K
2
SO
4
= 1.86 x 10 -4 moles x (174.3g/mole) = 0.0324g
%mass = (0.0324g/0.10g) *100 = 32% …… Na
2
SO
4
= 68%

Ionic substances in aqueous media dissociate (break apart) into their constituent ions. Ions that exist on both sides of an equation in identical forms can be eliminated and are called spectator ions.
So the equation:
K
2
SO
4
(aq) + BaCl
2
(aq)  2KCl(aq) + BaSO
4
(s)
In ionic form is written.
2K + + SO
42-
+ Ba 2+ + 2Cl -  2K + + 2Cl + BaSO
4
(s)
Notice that the polyatomic ions stay in tact charges must be included in the equation and the numbers of ions and the charge on both sides is consistent with balancing.
The solid is insoluble and does not dissociate.
Now eliminating the ions that appear on both sides of the equation
2K + + SO
42-
+ Ba 2+ + 2Cl -  2K + + 2Cl + BaSO
4
(s) leaving you with the net ionic equation:
Ba 2+ + SO
42-
 BaSO
4
(s)
This is a more correct view of what is happening. A barium ion bumps into a sulfate ion and forms an insoluble compound BaSO
4
(s) The rest of the ions just swim around and get dumped down the drain.