Question

which are the reactants in the following chemical equation? (select all that may apply) * 2 points
$ce{2h_{2} + o_{2} -> 2h_{2}o}$
$square$ $ce{h_{2}}$
$square$ $ce{o_{2}}$
$square$ $ce{h_{2}o}$
$square$ 2

baking a cake would be an example of what kind of change? * 1 point
$\bigcirc$ chemical
$\bigcirc$ physical
$\bigcirc$ both
$\bigcirc$ neither

rust is formed when iron reacts with the oxygen in moist air. suppose 100 g of iron metal rusts. we weigh the rust and find that the rust has a mass of 143 g. what mass of oxygen reacted with the iron? * 1 point
$\bigcirc$ 143g
$\bigcirc$ 43g
$\bigcirc$ 67g

Explanation:

First Question

In a chemical equation, reactants are on the left side of the arrow, the substances that undergo reaction. The coefficient (the number 2) is not a reactant, and $\text{H}_2\text{O}$ is a product on the right side.

Baking a cake involves irreversible chemical reactions (like protein denaturation, Maillard reactions) that create new substances, so it is a chemical change.

Step1: Apply Law of Conservation of Mass

The mass of products equals total mass of reactants. Let $m_{\text{O}_2}$ = mass of oxygen.
$\text{Mass of rust} = m_{\text{Fe}} + m_{\text{O}_2}$

Step2: Solve for oxygen mass

Rearrange to isolate $m_{\text{O}_2}$.
$m_{\text{O}_2} = \text{Mass of rust} - m_{\text{Fe}}$
$m_{\text{O}_2} = 143\ \text{g} - 100\ \text{g}$

Answer:

$\text{H}_2$, $\text{O}_2$

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Second Question