Question

1. classify classify the following changes as physical or chemical changes. a. water boils. b. salt dissolves in water. c. milk turns sour. d. a metal rusts. 37. explain according to the law of conservation of mass, when is mass conserved? 38. calculate hydrogen and oxygen react chemically to form water. how much water would form if 4.8 grams of hydrogen reacted with 38.4 grams of oxygen?

Explanation:

36.

• a. Water boiling is a physical change as it's a change of state from liquid to gas without changing the chemical composition of water ($H_2O$).
• b. Salt dissolving in water is a physical change. The salt ($NaCl$) separates into ions in water but no new substances are formed chemically.
• c. Milk turning sour is a chemical change. Bacteria convert lactose in milk into lactic acid, forming new substances.
• d. A metal rusting is a chemical change. Metals react with oxygen in the air to form metal - oxides, new chemical substances.

The law of conservation of mass states that mass is conserved in a closed system. In a chemical reaction or physical change, the total mass of the reactants is equal to the total mass of the products as long as no matter enters or leaves the system.

Step1: Write the chemical equation

The chemical equation for the reaction between hydrogen and oxygen to form water is $2H_2 + O_2
ightarrow2H_2O$.

Step2: Determine the molar masses

The molar mass of $H_2$ is approximately $2g/mol$, the molar mass of $O_2$ is approximately $32g/mol$, and the molar mass of $H_2O$ is approximately $18g/mol$.

Step3: Calculate the number of moles of reactants

The number of moles of $H_2$, $n_{H_2}=\frac{4.8g}{2g/mol} = 2.4mol$. The number of moles of $O_2$, $n_{O_2}=\frac{38.4g}{32g/mol}=1.2mol$.

Step4: Determine the limiting reactant

From the balanced equation, the mole - ratio of $H_2$ to $O_2$ is 2:1. For 1.2mol of $O_2$, we need $2\times1.2 = 2.4mol$ of $H_2$. Since we have 2.4mol of $H_2$, neither reactant is in excess.

Step5: Calculate the mass of water formed

From the balanced equation, the mole - ratio of $H_2$ to $H_2O$ is 1:1. So, the number of moles of $H_2O$ formed is 2.4mol. The mass of $H_2O$ formed, $m = n\times M=2.4mol\times18g/mol = 43.2g$.

Answer:

a. Physical change
b. Physical change
c. Chemical change
d. Chemical change

37.