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Conservation of Mass

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Conservation of Mass

A reaction vessel is being checked before a new process can run. Use the evidence and equations below to explain where the mass goes. Show working for calculations and include units.

The sealed-vessel test

A technician places 6.0 g of magnesium and 20.0 g of hydrochloric acid solution in a sealed flask. They weigh the complete flask and contents before and after the reaction. The reaction produces magnesium chloride and hydrogen gas.

1.Calculate the total mass of the reactants before they react. Include the correct unit.
2.The sealed flask and contents have the same mass after the reaction. Which statement best explains this result?
  • The atoms have disappeared, but the flask hides the change.
  • Atoms are rearranged into new substances; the total mass of the closed system is conserved.
  • The products must have less mass because a gas formed.
  • The reaction created extra atoms to replace the magnesium.
3.In an open flask, hydrogen gas escapes. The measured mass of the flask and remaining contents is lower after the reaction. Explain why this does not disprove conservation of mass.

Follow the atoms

A chemical equation represents a rearrangement of atoms. Coefficients count whole particles; subscripts show how many atoms of an element are in each particle.

4.Balance this equation using the smallest whole-number coefficients: __ H₂ + __ O₂ → __ H₂O. Then state how many hydrogen atoms and oxygen atoms are on each side.
5.A student changes H₂O to H₂O₂ to try to balance an equation. Explain why changing a subscript is not an acceptable way to balance it.
6.Balance the reaction of sodium bicarbonate with ethanoic acid: __ NaHCO₃ + __ CH₃COOH → __ CO₂ + __ H₂O + __ CH₃COONa. Check that each element has the same atom count on both sides.

Mass in the factory

A small production line makes iron from iron oxide and carbon monoxide. The balanced equation is Fe₂O₃ + 3CO → 2Fe + 3CO₂. The mass data below are from one run.

Material

Measured mass

Iron(III) oxide, Fe₂O₃

16.0 g

Carbon monoxide, CO

8.4 g

Iron, Fe

11.2 g

Carbon dioxide, CO₂

13.2 g

7.Calculate the total mass of the reactants and the total mass of the products in the mass record. Do the results support conservation of mass?
8.In another run, the reactants have a total mass of 32.5 g. The collected iron has a mass of 14.7 g. If all products are collected, what mass of carbon dioxide should form?
9.A group reports 10.0 g of reactants and 9.2 g of products from an open reaction vessel. Give one plausible explanation for the 0.8 g difference and name one improvement to the method.
10.A chemical plant uses conservation of mass to plan how much product a reaction can make. Explain how balancing equations and measuring mass help the plant reduce waste and check whether a process is working as expected.

3 printable pages

  • Conservation of Mass, page 1 of 3: The sealed-vessel test

    Page 1

  • Conservation of Mass, page 2 of 3: Follow the atoms, Mass in the factory

    Page 2

  • Conservation of Mass, page 3 of 3: 7. Calculate the total mass of the reactants and the total mass of the products in the…

    Page 3

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