There are 24 valance electrons in the compound.
<h3>What are valence electrons?</h3>
We know that the valence electrons are those that are found on the valence shell of an atom. There are four atoms that can be seen in the compound that is under consideration. The atoms are selenium, bromine (2 atoms) and oxygen.
Given that there are seven valence electrons in bromine and six valence electrons in oxygen and selenium. We have;
2(7 electrons of bromine) + 6 electrons of oxygen + 4 electrons of Se
= 24 electrons
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"The other halogens are not as electronegative and so other hydrogen halides cannot form hydrogen bonds between molecules. Only London Forces are formed. - Therefore more energy is required to break the intermolecular forces in HF than the other hydrogen halides and so it has a higher boiling point."
not a hack link, just stating where i got your answer from! -
https://www.mytutor.co.uk/answers/17558/A-Level/Chemistry/Explain-the-unusually-high-boiling-point-of-HF/
Definitely definite mass because liquids can take on any shape and can have different volumes.
From the statement of Hess' law, the enthalpy of the reaction A---> C is +90 kJ
<h3>What is Hess' law?</h3>
Hess' law of constant heat summation states that for a multistep reaction, the standard enthalpy of reaction is always constant and is independent of the pathway or intermediate routes taken.
From Hess' law, the enthalpy change for the reaction A ----> C is calculated as follows:
A---> C = A ---> B + B ---> C
ΔH of A---> C = 30 kJ + 60 kJ
ΔH = 90 kJ
Therefore, the enthalpy of the reaction A---> C is +90 kJ
The above reaction A---> C can be shown in the enthalpy diagram below:
A -------------------> C (ΔH = +90 kJ)
\ /
\ / (ΔH = +60 kJ)
(ΔH = +30 J) \ /
> B
Learn more about enthalpy and Hess law at: brainly.com/question/9328637
= 2 × 23 + 2 × 52 + 2 × 16
= 182 grams
1 mole of 
weighs = 182 g
8 moles weigh = 8× 182
=
or
