Question

A student investigated the law of conservation of mass.

The law of conservation of mass states that the mass of the products is equal to the mass of the reactants.

This is the method used.

1. Pour lead nitrate solution into a beaker labelled A.

2. Pour potassium chromate solution into a beaker labelled B.

3. Measure the mass of both beakers and contents.

4. Pour the solution from beaker B into beaker A.

5. Measure the mass of both beakers and contents again.

When lead nitrate solution and potassium chromate solution are mixed, a reaction takes place.

This is the equation for the reaction:

Pb(NO3)2(aq) + K2CrO4(aq) ⟶ PbCrO4(s) + 2KNO3(aq)

(a) What would the student see when the reaction takes place?

(1) (b) The table shows the student’s results.


Mass in g

Beaker A and contents before mixing 128.71

Beaker B and contents before mixing 128.97

Beaker A and contents after mixing 154.10

Beaker B after mixing 103.58

Show that the law of conservation of mass is true.

Use the data from the table above.
(2)

(c) What is the resolution of the balance used to obtain the results in the table?

Tick (✔) one box.


0.01 g 0.1 g 1 g 100 g

Answer 1

$\bold\pink{Answer}$

Reaction of potassium chromate with lead nitrate results in formation of yellow precipitate lead chromate PbCrO4:

K2CrO4 + Pb(NO3)2 = PbCrO4↓ + 2KNO3

In the past lead chromate is used as a yellow pigment in paints. For example, it has been used in the paint to color school buses. But because of safety issues (it contains two toxic metals: lead and hexavalent chromium), this salt is being banned.

Answer 2

Answer: (a) formation of a solid precipitate (PbCrO4)

              (b)  mass of reactants = 257.68g

                   mass of products = 257.68g

              (c) 0.01 g

Explanation:

The law of conservation of mass states that

“The mass in an isolated system can neither be created nor be destroyed but can be transformed from one form to another”.

(a) The student would see the formation of a solid precipitate (PbCrO4) when the lead nitrate and potassium chromate solutions are mixed.

(b) To show that the law of conservation of mass is true, we can add the mass of the reactants (beaker A and contents before mixing + beaker B and contents before mixing) and compare it to the mass of the products (beaker A and contents after mixing + beaker B after mixing).

128.71g + 128.97g = 257.68g (mass of reactants)

154.10g + 103.58g = 257.68g (mass of products)

As we can see, the mass of the reactants is equal to the mass of the products, so the law of conservation of mass is true.

(c) 0.01 g

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