TUTORIAL – Lab: Experiment 5 Practice Questions

Question 1: Phosphate ions can be removed from wastewater in sewage treatment by precipitation with aluminum (III) cations. Write the formula of the precipitate formed.

Answer to Question 1:
Aluminum forms Al³⁺ ions.
Phosphate ions: PO₄^3-

Chemical formula formed from these ions is AlPO₄

Review Unit 4 section 4.1 (http://nobel.scas.bcit.ca/courses/wpmu/chem0011/unit-4/)

Question 2: One cup of a particular clear chicken broth yields 4.308 g of silver chloride when excess silver nitrate is added to it. Assuming that all the chloride comes from sodium chloride, what is the mass of sodium chloride in the sample of chicken broth?

Answer to Question 2:
From the question, we are able to gather the following information:

1. Chicken broth contains sodium chloride, NaCl.
2. The NaCl content in chicken broth can be determined by doing a chemical reaction. The reaction involves adding silver nitrate to the chicken broth.
3. A chemical reaction occurs and the product of the chemical reaction is silver chloride, AgCl.
4. The mass of AgCl formed is given. It is 4.308 g.
5. From the mass of AgCl, the question asks to calculate the mass of NaCl in one cup of chicken broth.

Explanation of the chemical reaction
Silver chloride, AgCl, is the product of a reaction between silver nitrate, AgNO₃ and chloride ions, Cl^–. The source of Cl^– ions is from sodium chloride, NaCl, that is in the chicken broth.

This is how chemists represent the chemical reaction that is described above.

AgNO₃ (aq) + Cl^– (aq) AgCl (s) + NO₃^– (aq)

(Note: At this point in the course, I do not expect you to be able to write the chemical equation.)

The formation of AgCl (i.e. 4.308 g AgCl) requires a source of Cl^– ions. The source of Cl^– ions is from the NaCl in the sample of the chicken broth. Therefore, from the mass of AgCl, we would be able to calculate the mass of NaCl in the chicken broth.

The following steps are used in this calculation:
Step 1: Calculate the molar mass of AgCl.
It is is 143.321 g/mole.

Step 2: From 4.308 g of AgCl, calculate the number of moles of AgCl.
4.308 g x (^{1 mole}/_{143.321 g}) = 0.030058 mole AgCl
(Note: Carry more digits because this is an intermediate step in the calculation)

Step 3: From 0.030058 mole AgCl, calculate the number of moles of Cl^– ions.
0.030058 mole AgCl x (^{1 mole Cl– ion}/_{1 mole AgCl}) = 0.030058 mole Cl^– ions

Step 4: From 0.030058 mole Cl^– ions, calculate the number of moles of NaCl.
0.030058 mole Cl^– ions x (^{1 mole NaCl}/_{1 mole Cl^– ions}) = 0.030058 mole NaCl

Step 5: From 0.030058 mole NaCl, calculate the mass of NaCl.
0.030058 mole NaCl x (^{58.443 g}/_{1 mole NaCl}) = 1.757 g NaCl (4 significant figures)

The chicken broth sample has 1.757 g NaCl.

Review Unit 5 section 5.1, 5.2, 5.3, 5.5 and 5.6 (http://nobel.scas.bcit.ca/courses/wpmu/chem0011/unit-5/)