I would like to have the translation (in context and word by word) of the expressions "emptio spei" and "emptio rei speratae", which are used in Law as types of contracts; emptio spei is the type of contract in which you purchase something unsure, something like a trip to see aurora borealis (it doesn't matter if it was raining on that day). The emptio rei speratae is a kind of contract in which only the amount of the purchased item is unknown beforehand.
Emptio is a purchase—either the act of purchasing or the thing purchased. It's from the stem of the verb emō 'to buy', plus the action noun suffix -tiō. The -p- is epenthetic, and a regular development between m and t.
Spei is the genitive of spēs 'hope; anticipation'. It's an objective genitive, denoting the thing that is bought. Emptio spei literally means 'purchase of [a] hope' or 'purchase of an anticipation'.
Rei speratae is likewise an objective genitive: rēs 'thing' is the noun, and spērātus, the adjective accompanying it (in the feminine because rēs is feminine), is the perfect passive participle of spērō 'to hope; to await', obviously related to spēs. Emptio rei speratae means 'purchase of a thing hoped for' or 'purchase of an awaited thing'.
The catch here is that Latin doesn't distinguish between merely hoping for something and actively awaiting it, which illustrates the problem of trying to interpret Law Latin jargon by simply translating it.