Answer:
So, in 1 mol there are 6.02x10^23 atoms, regardless of the susbtance, only the weight of the mol changes. Doing a simple rule of 3 we can get many of the answers to your questions.
Step-by-step explanation:
1mol of Ar → 6.02x10^23 atoms
0.25 mol → X
So now what we do is (0.25mol * 6.02x10^23 atoms)/1 mol = X
That gives us 1.505x10^23 atoms in 0.25 mol of Ar.
For this I'm guessing you mean the stable form of oxygen, O2. The Avogadro constant stays well, constant.
1 mol of O2 → 6.02x10^23 molecules
1.0×10^-4 mol of O2 → X molecules
So you can do (1.0×10^-4 * 6.02x10^23) / 1mol = X
(I feel like this is homework, so i will help you, but not give you all the answer, will be good practice to try yourself) :)
For this you need to know the weight of a mol of Be. Luckily the periodic table gives us that number. The number under each element is the weight in grams of 1 mol .