Q: A student combines 10.0 mL of 0.010 M $$\mathrm{Na}_3\mathrm{PO}_4$$ with 20.0 mL of 0.010 M $$\mathrm{Co(NO}_3)_2. $$Calculate the moles of $$\mathrm{Na}_3\mathrm{PO}_4$$ used.

$$0.010 \; \mathrm{mol/L} \times 0.010 \; \mathrm{L} = 0.00010$$ mol

Q: Calculate the theoretical yield (in grams) of $$\mathrm{Co}_3(\mathrm{PO}_4)_2$$ formed when 0.00200 mol of $$\mathrm{Na}_3\mathrm{PO}_4$$ reacts with excess $$\mathrm{Co(NO}_3)_2. ($$Molar mass of $$\mathrm{Co}_3(\mathrm{PO}_4)_2 = 366.8 g/$$mol)

$$0.00200 \times \frac{1}{2} \times 366.8 = 0.367 g$$

Q: If the actual yield of $$\mathrm{Co}_3(\mathrm{PO}_4)_2 in an $$experiment is 0.0175 g and the theoretical yield is 0.0200 g, what is the percent yield?

$$\frac{0.0175}{0.0200} \times 100 = 87.5\%$$

Q: A student mixes 4.00 mL of 0.095 M $$\mathrm{Na}_3\mathrm{PO}_4$$ with 20.0 mL of 0.100 M $$\mathrm{Co(NO}_3)_2. $$Calculate the moles of $$\mathrm{Co(NO}_3)_2$$ used.

$$0.100 \times 0.0200 = 0.00200$$ mol

Question 1

Which of the following best describes the role of the limiting reagent in a precipitation reaction?

Accepted Answer

It is the reagent that is completely consumed during the reaction and determines the maximum amount of product formed.

Question 2

Given the reaction: $$2\mathrm{Na}_3\mathrm{PO}_4(aq) + 3\mathrm{Co(NO}_3)_2(aq) \rightarrow \mathrm{Co}_3(\mathrm{PO}_4)_2(s) + 6\mathrm{NaNO}_3(aq)$$, if you mix 0.020 mol of $$\mathrm{Na}_3\mathrm{PO}_4$$ with 0.030 mol of $$\mathrm{Co(NO}_3)_2$$, which reagent is limiting?

Accepted Answer

Neither, both are present in the exact stoichiometric ratio

Question 3

A student combines 10.0 mL of 0.010 M $$\mathrm{Na}_3\mathrm{PO}_4$$ with 20.0 mL of 0.010 M $$\mathrm{Co(NO}_3)_2. $$Calculate the moles of $$\mathrm{Na}_3\mathrm{PO}_4$$ used.

Accepted Answer

$$0.010 \; \mathrm{mol/L} 	imes 0.010 \; \mathrm{L} = 0.00010$$ mol

Question 4

In the experiment, after mixing $$\mathrm{Na}_3\mathrm{PO}_4$$ and $$\mathrm{Co(NO}_3)_2$$, a solid forms. According to solubility rules, what is the identity of this solid?

Accepted Answer

$$\mathrm{Co}_3(\mathrm{PO}_4)_2$$

Question 5

If 0.0100 mol of $$\mathrm{Na}_3\mathrm{PO}_4$$ reacts with excess $$\mathrm{Co(NO}_3)_2$$, how many moles of $$\mathrm{Co}_3(\mathrm{PO}_4)_2$$ can theoretically be formed? (Use the balanced equation: $$2\mathrm{Na}_3\mathrm{PO}_4 + 3\mathrm{Co(NO}_3)_2 \rightarrow \mathrm{Co}_3(\mathrm{PO}_4)_2 + 6\mathrm{NaNO}_3)$$

Accepted Answer

$$0.00500$$ mol

Question 6

A mixture of $$\mathrm{Na}_3\mathrm{PO}_4$$ and $$\mathrm{Co(NO}_3)_2 is $$filtered after reaction. The supernatant is tested with more $$\mathrm{Na}_3\mathrm{PO}_4$$ and a precipitate forms. What does this indicate about the original mixture?

Accepted Answer

$$\mathrm{Co(NO}_3)_2$$ was the limiting reagent

Question 7

Calculate the theoretical yield (in grams) of $$\mathrm{Co}_3(\mathrm{PO}_4)_2$$ formed when 0.00200 mol of $$\mathrm{Na}_3\mathrm{PO}_4$$ reacts with excess $$\mathrm{Co(NO}_3)_2. ($$Molar mass of $$\mathrm{Co}_3(\mathrm{PO}_4)_2 = 366.8 g/$$mol)

Accepted Answer

$$0.00200 	imes \frac{1}{2} 	imes 366.8 = 0.367 g$$

Question 8

If the actual yield of $$\mathrm{Co}_3(\mathrm{PO}_4)_2 in an $$experiment is 0.0175 g and the theoretical yield is 0.0200 g, what is the percent yield?

Accepted Answer

$$\frac{0.0175}{0.0200} 	imes 100 = 87.5\%$$

Question 9

Why does the plot of theoretical mass of $$\mathrm{Co}_3(\mathrm{PO}_4)_2 (y-$$axis) vs. mass of $$\mathrm{Na}_3\mathrm{PO}_4 (x-$$axis) plateau at higher masses of $$\mathrm{Na}_3\mathrm{PO}_4$$?

Accepted Answer

Because $$\mathrm{Na}_3\mathrm{PO}_4$$ becomes the excess reagent and $$\mathrm{Co(NO}_3)_2 is $$limiting

Question 10

Which of the following best explains why the actual yield of $$\mathrm{Co}_3(\mathrm{PO}_4)_2 is $$often less than the theoretical yield in a real experiment?

Accepted Answer

Some product is lost during filtration and transfer

Question 11

A student mixes 4.00 mL of 0.095 M $$\mathrm{Na}_3\mathrm{PO}_4$$ with 20.0 mL of 0.100 M $$\mathrm{Co(NO}_3)_2. $$Calculate the moles of $$\mathrm{Co(NO}_3)_2$$ used.

Accepted Answer

$$0.100 	imes 0.0200 = 0.00200$$ mol

Question 12

In the reaction $$2\mathrm{Na}_3\mathrm{PO}_4 + 3\mathrm{Co(NO}_3)_2 \rightarrow \mathrm{Co}_3(\mathrm{PO}_4)_2 + 6\mathrm{NaNO}_3$$, what is the mole ratio of $$\mathrm{Na}_3\mathrm{PO}_4 to$$ $$\mathrm{Co}_3(\mathrm{PO}_4)_2$$?

Accepted Answer

2:1

Question 13

After filtering the reaction mixture, the supernatant is tested with $$\mathrm{Co(NO}_3)_2$$ and no precipitate forms. What does this indicate?

Accepted Answer

All $$\mathrm{Na}_3\mathrm{PO}_4$$ was consumed and $$\mathrm{Co(NO}_3)_2 is in $$excess

Question 14

Which of the following ions are considered spectator ions in the reaction between $$\mathrm{Na}_3\mathrm{PO}_4$$ and $$\mathrm{Co(NO}_3)_2$$?

Accepted Answer

$$\mathrm{Na}^+$$ and $$\mathrm{NO}_3^-$$

Question 15

If a student accidentally uses twice the amount of $$\mathrm{Na}_3\mathrm{PO}_4$$ required for complete reaction with $$\mathrm{Co(NO}_3)_2$$, what will be observed in the supernatant after filtration?

Accepted Answer

Excess $$\mathrm{Na}_3\mathrm{PO}_4$$ ions will be present

Question 16

Which calculation would you use to determine the moles of $$\mathrm{Co(NO}_3)_2$$ needed to react with a given amount of $$\mathrm{Na}_3\mathrm{PO}_4$$?

Accepted Answer

Moles of $$\mathrm{Na}_3\mathrm{PO}_4 	imes \frac{3}{2}$$

Question 17

A student observes that the mass of $$\mathrm{Co}_3(\mathrm{PO}_4)_2$$ precipitate is less than the theoretical yield. Which of the following is NOT a likely reason?

Accepted Answer

All product was perfectly recovered

Precipitation Reactions and Limiting Reagents: Laboratory Study Guide

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