HS1-PS1-7: Conservation of Mass

Which statement best describes the law of conservation of mass?

• A) The total mass of reactants is always greater than the total mass of products

• B) Mass is created in chemical reactions

• C) The total mass of reactants equals the total mass of products ✅

• D) Atoms can disappear during a chemical reaction

Why must chemical equations be balanced?

• A) To show that energy is conserved

• B) To follow the law of conservation of mass ✅

• C) To create new elements

• D) To increase the amount of product

Which of the following equations is correctly balanced?

• A) H2 + O2 --> H2O

• B) CH4 + O2 --> CO2 + H2O

• C) H2 + O2 --> H2O2 ✅

• D) 2Na + Cl2 --> 2NaCl2

In the reaction C3H8 + O2 --> H2O + CO2, what coefficient must be placed in front of O2 to balance the equation?

• A) 3

• B) 4

• C) 5 ✅

• D) 6

Which type of reaction always follows the law of conservation of mass?

• A) Combustion

• B) Synthesis

• C) Decomposition

• D) All of the above ✅

How does the number of atoms of each element in the reactants compare to the number in the products?

• A) There are more atoms in the reactants

• B) The number of atoms is always equal ✅

• C) Atoms disappear in the products

• D) Atoms are created in the products

If 44 g of CO₂ reacts completely with 18 g of H₂O, what is the total mass of the products?

• A) 44 g

• B) 62 g ✅

• C) 26 g

• D) Cannot be determined

A student starts with 20 grams of reactants in a sealed container. What should be the total mass of the products?

• A) 10 grams

• B) 20 grams ✅

• C) 30 grams

• D) It depends on the reaction

A balanced equation for a reaction must have the same number of which particles on both sides?

• A) Protons

• B) Atoms ✅

• C) Electrons

• D) Molecules

Which of the following balanced equations demonstrates conservation of mass?

• A) N2 + 3H2 --> 2NH3 ✅

• B) 2H2 + O2 --> 2H2O2

• C) C + O2 --> CO

• D) Fe + O2 --> Fe2O3