Hint: The compounds having the suffixes -one is those compounds which have double bonded oxygen on the carbon. This is also called the keto compound. The two bonds present are pi and sigma bonds. If two hydroxide ions are attached to two carbon chains which are adjacent to each other in an acidic medium, then water gets dehydrated.
Complete answer: We have been given that we need to convert but- $2$-yne to butane-$2$-one. but- $2$-yne is an unsaturated hydrocarbon which says that the triple bond which is denoted by the suffix -yne, is present on the second carbon of the chain. The prefix but- tells us about the number of carbons that are four carbons. It is a straight chain and we have to convert this into but-$2$-one. Here the keto compounds are also present on the second carbon. Now there is a reagent called Raskolnikov reagent which contains mercury ions in an acidic medium of hydrogen sulphuric acid. The reaction of this reagent with unsaturated hydrocarbon produced a ketone. In this reaction what happens is that double bonds present between the two unsaturated carbon having triple bond break and what happens is that two hydroxide ion gets attached to the two carbons and only single bond is left between the carbon. The medium is acidic and the water gets dehydrated and there is a double bonded oxygen present which is a keto group. So, we can convert but-$2$-one from but- $2$-yne by using mercury sulphate and hydrogen sulphate.
Note: Conversion reaction is very important for the conversion while studying hydrocarbons. This reagent is also used for the conversion of alkenes into alkanol. What happens is that the pi bond in the double bond breaks and more electropositive carbon of the two bonds gets attached with the hydroxide ion.