Solubility, Stoichiometry, and Spectator Ions

Objectives

• Apply stoichiometry to identify an unknown metal sample• Use solubility rules to perform a titration

Introduction• Solubility rules can be used to determine which ionic compounds are soluble and

are not soluble in water.• Stoichiometry is a quantitative relationship between substances taking part in a

reaction or in the formation of a compound.• Study of the relationships between the amount of reactants and products on the

macroscopic scale

�𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁 𝑜𝑜𝑜𝑜 𝑎𝑎𝑎𝑎𝑜𝑜𝑁𝑁𝑎𝑎 𝑜𝑜𝑜𝑜𝑎𝑎𝑡𝑁𝑁 𝑁𝑁𝑁𝑁𝑎𝑎𝑟𝑟𝑎𝑎𝑎𝑎𝑜𝑜𝑎𝑎 𝑎𝑎𝑠𝑠𝑠𝑠𝑁𝑁

= �𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁 𝑜𝑜𝑜𝑜 𝑎𝑎𝑎𝑎𝑜𝑜𝑁𝑁𝑎𝑎 𝑜𝑜𝑜𝑜𝑎𝑎𝑡𝑁𝑁 𝑝𝑝𝑁𝑁𝑜𝑜𝑠𝑠𝑁𝑁𝑟𝑟𝑎𝑎 𝑎𝑎𝑠𝑠𝑠𝑠𝑁𝑁

• In this experimento you will titrate your metal sample using strong bases (NaOH and KOH).o The equivalence point is the point, of a chemical reaction is the point at which

chemically equivalent quantities of metal ion and base have been mixed.

Moles and Molarity

Mole• Unit used by chemists when counting numbers of atoms or molecules in a

sample• One mole of any element contains 6.02214 x 1023 atoms of that element.

Molarity• Moles of solute per liter of solution

Molarity (M) = 𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀 𝑀𝑀𝑜𝑜 𝑀𝑀𝑀𝑀𝑀𝑀𝑠𝑠𝑠𝑠𝑀𝑀 (𝑚𝑚𝑀𝑀𝑀𝑀)1 𝐿𝐿 𝑀𝑀𝑜𝑜 𝑀𝑀𝑀𝑀𝑀𝑀𝑠𝑠𝑠𝑠𝑠𝑠𝑀𝑀𝑠𝑠 (𝑀𝑀)

• Conversion factor that transforms units of volume to moles and vice versa.

Part A: Titration

• Identify charge of metal (You will determine if your unknown has Al3+ or Zn2+ ) o Metals react with hydroxide to form precipitateo Moves the solution basic by removing metals

AlCl3 aq + 3 NaOH aq → Al(OH)3 s + 3 NaCl (aq)

Zn NO3 2 aq + 2 NaOH aq → Zn OH 2 s + 2NaNO3(aq)• Diluted unknown metal compound

o M1V1=M2V2

o M = Molarityo V = Volume

Part A: Titration

• How will you know when all the metal ions in your unknown solution have reacted?

• Equivalence point• We can also calculate a guess.

DON’T FORGET TO EMAIL YOURSELF THE DATA!

http://www.sparknotes.com/chemistry/acidsbases/titrations/problems.html

Part A: Titration

• Obtain 1 mL of a 0.5 M unknown metal solution.• You will be titrating with two separate solutions.• One time through steps 1-7 using NaOH and one time through

steps 1-7 using KOH.• Set up LabQuest 3 to record a pH vs. volume analysis

Part A: Titration

• Why is the titration performed with the buret?o Make sure you properly are reading the buret

• Make sure to keep solution stirring

Part A: Drop Counter Setup

Go Direct Drop Counter

Go Direct pH Probe

Buret

Stir Bar

Test drop counter : LED should light up as each drop passes through. Must be “dropwise” for system to count each drop.

Calculations• Determine the amount of sodium hydroxide used to reach pH = 7

𝑁𝑁𝑚𝑚 𝑁𝑁𝑎𝑎𝑁𝑁𝑁𝑁1000 𝑁𝑁𝑚𝑚

×0.1 𝑁𝑁𝑜𝑜𝑚𝑚𝑁𝑁 𝑁𝑁𝑎𝑎𝑁𝑁𝑁𝑁

1𝑚𝑚= 𝑁𝑁𝑜𝑜𝑚𝑚𝑁𝑁𝑎𝑎 𝑁𝑁𝑎𝑎𝑁𝑁𝑁𝑁 𝑎𝑎𝑠𝑠𝑠𝑠𝑁𝑁𝑠𝑠

• Determine the number of moles of your unknown

1 𝑁𝑁𝑚𝑚 𝑁𝑁𝑜𝑜𝑢𝑢𝑜𝑜𝑜𝑜𝑢𝑢𝑜𝑜 𝑁𝑁𝑁𝑁𝑎𝑎𝑎𝑎𝑚𝑚1000 𝑁𝑁𝑚𝑚 ×

0.5 𝑁𝑁𝑜𝑜𝑚𝑚𝑁𝑁 𝑁𝑁𝑜𝑜𝑢𝑢𝑜𝑜𝑜𝑜𝑢𝑢𝑜𝑜 𝑁𝑁𝑁𝑁𝑎𝑎𝑎𝑎𝑚𝑚1 𝑚𝑚 = 𝑁𝑁𝑜𝑜𝑚𝑚𝑁𝑁𝑎𝑎 𝑁𝑁𝑜𝑜𝑢𝑢𝑜𝑜𝑜𝑜𝑢𝑢𝑜𝑜 𝑁𝑁𝑁𝑁𝑎𝑎𝑎𝑎𝑚𝑚

• Determine the charge of your unknown metal by calculating the ratio of moles of unknown metal to moles of sodium hydroxide

𝑁𝑁𝑜𝑜𝑚𝑚𝑁𝑁𝑎𝑎 𝑜𝑜𝑜𝑜 𝑁𝑁𝑎𝑎𝑁𝑁𝑁𝑁𝑁𝑁𝑜𝑜𝑚𝑚𝑁𝑁𝑎𝑎 𝑜𝑜𝑜𝑜 𝑁𝑁𝑜𝑜𝑢𝑢𝑜𝑜𝑜𝑜𝑢𝑢𝑜𝑜 𝑁𝑁𝑁𝑁𝑎𝑎𝑎𝑎𝑚𝑚 = 𝑟𝑟𝑡𝑎𝑎𝑁𝑁𝑐𝑐𝑁𝑁 𝑜𝑜𝑜𝑜 𝑦𝑦𝑜𝑜𝑁𝑁𝑁𝑁 𝑁𝑁𝑁𝑁𝑎𝑎𝑎𝑎𝑚𝑚

• Repeat for potassium hydroxide

Solubility

• Record observations:o You will need to know which test

formed a precipitate

• Solubility rules will help you determine if your unknown’s anion is nitrate or chloride.

𝐴𝐴𝑚𝑚𝐶𝐶𝑚𝑚3 𝑎𝑎𝑎𝑎 + 3 𝐴𝐴𝑐𝑐𝑁𝑁𝑁𝑁3 𝑎𝑎𝑎𝑎 → 3 𝐴𝐴𝑐𝑐𝐶𝐶𝑚𝑚 𝑎𝑎 + 𝐴𝐴𝑚𝑚(𝑁𝑁𝑁𝑁3)3(𝑎𝑎𝑎𝑎)

𝐴𝐴𝑚𝑚(𝑁𝑁𝑁𝑁3)3 𝑎𝑎𝑎𝑎 + 𝐴𝐴𝑐𝑐𝑁𝑁𝑁𝑁3 𝑎𝑎𝑎𝑎 → 𝐴𝐴𝑚𝑚(𝑁𝑁𝑁𝑁3)3 𝑎𝑎𝑎𝑎 + 𝐴𝐴𝑐𝑐𝑁𝑁𝑁𝑁3(𝑎𝑎𝑎𝑎) (No precipitation)

Part C: Drying Precipitate

• You are doing part C to confirm your results from part B.• Centrifuge procedure

o Test tubes need to be “mass balanced” in the centrifuge.o Test tubes need to have the same volume. o Test tubes need to also be symmetrical when loaded.o Please note how test tubes are in black sleeves.o If loaded improperly, test tubes could break.o Speed may need to be adjusted if no solids collect at

bottom.

Part C: Drying Precipitate

Hazards and Waste

Hazards• Be careful of broken glass • Silver nitrate will stain your skin

Waste• Solution contains hydroxide go

to Base Waste• Ethanol goes to Organic Waste