General Chemistry Practice

Air is a homogeneous mixture because its composition is fairly uniform throughout.

Salt water is also a homogeneous mixture because the salt is dissolved evenly in the water.

Granite is a heterogeneous mixture because its different minerals remain visible as separate parts.

The atomic number gives the number of protons, so the atom has $12$ protons.

The mass number is

so

which gives $N = 12$ neutrons.

Because the atom is neutral, it also has $12$ electrons.

Group 2 elements usually have $2$ valence electrons.

Group 18 elements are especially stable because their outer electron shells are full.

A $p$ subshell holds $6$ electrons.

A $d$ subshell holds $10$ electrons.

Across a period, atomic radius decreases.

Across a period, electronegativity generally increases.

The best Lewis structure is

Carbon has four valence electrons and four bonding pairs around it, so its formal charge is $0$.

Each oxygen has two lone pairs and one double bond, which also gives a formal charge of $0$.

So every atom in the preferred structure has formal charge $0$.

Oxygen has four electron groups around it: two bonding pairs and two lone pairs.

That gives a tetrahedral electron geometry and a bent molecular shape.

Because the bond dipoles do not cancel in a bent molecule, $H_2O$ is polar.

$\mathrm{NaCl}$ is sodium chloride.

$\mathrm{CO_2}$ is carbon dioxide.

$\mathrm{HCl(aq)}$ is hydrochloric acid.

$\mathrm{CaCO_3}$ is calcium carbonate.

Start by balancing carbon and hydrogen:

Now count oxygen atoms on the product side: there are $4$ oxygen atoms total, so the reactant side needs $2$ oxygen molecules.

The balanced equation is

This is a combustion reaction because a hydrocarbon reacts with oxygen to form carbon dioxide and water.

Use Avogadro's number:

Rounded to three significant figures, the answer is

The molar mass of water is $18.0$ g/mol, so

Now convert moles to molecules:

The reaction needs $2$ mol of $H_2$ for every $1$ mol of $O_2$.

If $2.0$ mol of $O_2$ are available, the reaction requires

Since $5.0$ mol of $H_2$ are available, hydrogen is in excess and oxygen is limiting.

From the balanced equation, $1$ mol of $O_2$ produces $2$ mol of $H_2O$, so

Use the ideal gas law:

Solve for volume:

Substitute the values:

Rounded appropriately,

Use the dilution relationship:

Solve for the stock volume:

Use

Substitute the values:

Because the temperature increases, the heat is positive.

Compare $Q$ and $K$.

Since

the system has too few products relative to equilibrium.

The reaction shifts to the right, toward ammonia, to increase $Q$ until equilibrium is reached.

Use the Henderson-Hasselbalch equation:

Substitute the ratio:

The rate changes by the factor

So the rate increases by a factor of $4.5$.

The outside solution is hypotonic relative to the cell.

Water moves into the cell by osmosis.

The cell swells and may burst if the concentration difference is large enough.

Use Dalton's law:

So

Rounded appropriately,

Use

Substitute the values:

Because

the protonated form predominates.

That is the key pH-to-pKa rule for weak acids.

Loss of mass means oxidation is happening there.

Oxidation occurs at the anode, so that electrode is the anode.

The electrons flow away from the anode and toward the cathode.

Water is polar, so ionic and polar substances dissolve better than nonpolar substances.

$\mathrm{NaCl}$ is ionic, so it is the most water-soluble.

$\mathrm{NH_3}$ is polar and can hydrogen bond, so it is next.

$\mathrm{CH_4}$ is nonpolar, so it is the least soluble.

So the ranking is

Removing product lowers the product concentration, so $Q$ becomes less than $K$.

To restore equilibrium, the reaction shifts to the right and makes more product.

The added acid reacts with the base form:

So the buffer amounts become:

Use Henderson-Hasselbalch:

From the balanced equation, $1$ mol of $O_2$ produces $2$ mol of water.

So to make $2.00$ mol of $H_2O$, you need

Now use the ideal gas law: