Final answer:
An alkane with the formula C7H12 has two rings. The general formula for alkanes, CnH2n+2, indicates that a C7 alkane should have 16 hydrogens. The deficit of four hydrogens (compared to C7H16) suggests the presence of two rings.
Step-by-step explanation:
To determine how many rings an alkane with the formula C7H12 has, we use the general formula for alkanes: CnH2n+2. For an alkane with no rings or double bonds (fully saturated), the correct formula is CnH2n+2. A C7 alkane would normally have 2(7)+2 = 16 hydrogens if it were fully saturated and ring-free. Since the provided formula has only 12 hydrogens, there are insufficient hydrogens for it to be a straight-chain alkane, which indicates the presence of rings or double bonds.
The difference in the number of hydrogens (16 - 12 = 4) suggests that for each ring or double bond present, two hydrogens are missing from the alkane formula (as per the rule of saturated hydrocarbons). Hence, to account for a difference of four hydrogens, the C7H12 molecule must have two rings or double bonds. Because the question specifies alkanes and alkanes do not have double bonds, we can conclude that there are two rings present in the alkane molecule.