Determine the molecular formula of compound P.

Heptane ($C_7H_{16}$) is cracked into a FIVE CARBON alkane P and compound Q.
The general formula for an alkane is $C_nH_{2n+2}$. For a five-carbon alkane, $n=5$.
So, compound P is $C_5H_{2(5)+2} = C_5H_{12}$.

The total number of carbon atoms must be conserved: $7 = 5 + C_Q \implies C_Q = 2$.
The total number of hydrogen atoms must be conserved: $16 = 12 + H_Q \implies H_Q = 4$.
Therefore, the molecular formula for compound Q is $C_2H_4$.

The molecular formula for compound Q is $\boxed{\text{C}_2\text{H}_4}$.

4.1.3
The most important use of alkanes is as fuels.

4.1.4

Complete combustion of a hydrocarbon produces carbon dioxide ($CO_2$) and water ($H_2O$).

There are 5 carbon atoms on the left, so there must be 5 $CO_2$ molecules on the right.

There are 12 hydrogen atoms on the left, so there must be 6 $H_2O$ molecules on the right ($6 \times 2 = 12$).

On the right side, there are $(5 \times 2) + (6 \times 1) = 10 + 6 = 16$ oxygen atoms.
To balance this, there must be 8 $O_2$ molecules on the left ($8 \times 2 = 16$).

The balanced equation for the complete combustion of compound P is:

4.2.1
The flow diagram shows a straight chain secondary haloalkane reacting to form a 5-carbon alcohol (Alcohol P), which then undergoes elimination to form $C_5H_{10}$ (a 5-carbon alkene). This means the haloalkane must also have 5 carbon atoms. A straight chain secondary haloalkane with 5 carbons would have the halogen on the second or third carbon. Given the handwritten notes on the left, the halogen is on the second carbon. Since NaBr is a product of Reaction 1, the halogen is bromine.
The IUPAC name is 2-bromopentane.

4.2.2
Reaction 1 is the conversion of a haloalkane to an alcohol. This is a hydrolysis reaction (or nucleophilic substitution).

4.2.3
Besides heat, another reaction condition for the hydrolysis of a haloalkane is an aqueous strong base (e.g., NaOH(aq) or KOH(aq)).

4.2.4
If the secondary haloalkane is 2-bromopentane, then Alcohol P, formed by replacing the bromine with a hydroxyl group, is pentan-2-ol.
The structural formula of alcohol P is:

4.2.5
Reaction 2 is the conversion of an alcohol to an alkene ($C_5H_{10}$). This is an elimination reaction where water is removed from the alcohol.
The name of this type of elimination reaction is dehydration.

4.2.6
Alcohol P is pentan-2-ol ($CH_3CH(OH)CH_2CH_2CH_3$). During dehydration, the -OH group and a hydrogen atom from an adjacent carbon are removed to form a double bond. According to Zaitsev's rule, the major product is the more substituted alkene.
Removing H from C1 gives pent-1-ene ($CH_2=CHCH_2CH_2CH_3$).
Removing H from C3 gives pent-2-ene ($CH_3CH=CHCH_2CH_3$).
Pent-2-ene is the more substituted alkene and thus the major product.
The structural formula for the product that was produced (pent-2-ene) is: