C4H9OH is the substrate, Br- is the nucleophile and H2O is the leaving group.
In an SN1 reaction, the rate limiting step is the formation of the carbocation. The carbocation is formed as water which is the leaving group departs. After the leaving group departs, the nucleophile (Br-) attacks to yield the major organic product. The protonation of the -OH group is the first step in the reaction.
It then follows that, C4H9OH is the substrate, Br- is the nucleophile and H2O is the leaving group.
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Answer:
New volume is 25.0 mL
Explanation:
Let's assume the gas sample behaves ideally.
According to combined gas law for an ideal gas-

where,
and
represent initial and final pressure respectively
and
represent initial and final volume respectively
and
represent initial and final temperature (in kelvin) respectively
Here,
,
,
and 
So, 
So, the new volume is 25.0 mL
Protons-19 because protons are equal to the atomic number which is 19 , electrons-19 electrons are equal to number of protons, neutrons-20 because the mass number of potassium is 39 so subtract 39 from 19 (atomic number) which gives us 20