Tell me the mass number of an atom with 4 protons, 5 neutrons, and 2 electrons.

I'm pretty sure your answer will be 9

9

Explanation:

Mass number = proton + neutron

                       = 4 + 5

                       = 9

1. 1 mole of anything contains 6.022x10^23 pieces  

d  

2. a  

3. c, number  

5. 1.24x10^24atoms / 6.022x10^23atoms/mole = 2moles h2 atoms  

6. fe2br3 mass = 2moles x mass fe + 3 moles x mass br  

molar mass = grams / number of moles = g/mole  

to find moles = g / molar mass  

to find mass = g = molar mass / moles