Answer: The answer is 6.78 grams.
Explanation: The equation used for solving this type of problems is:

where,
is the initial amount of radioactive substance, N is the remaining amount and n is the number of half lives.
Number of half lives is calculated on dividing the given time by the half life.
n = time/half life
Time is given as 48.0 hours and the half life is given as 4.536 days. let's make the units same and for this let's convert the half life from days to hours.

= 108.864 hours
So,
= 0.441
Since 5.00 g is the required amount when the radioactive substance is delivered to the scientist, it would be the final amount that is N. We need to calculate the initial amount. Let's plug in the values in the equation:



= 6.78 g
So, 6.78 g of the radioactive substance needs to be ordered.
Answer:
The red ball has a greater velocity than the purple ball
Explanation:
The law of conservation of momentum states that, if left alone, the total momentum of two interacting objects that make up a system. which that system remains the same
Answer:
It takes 7 mol H2 to produce 7 mol H2O
Explanation:
In 2H2O, there's 2 moles of H2 for every 1 mol O2.
So, if oxygen is limiting you'd only be able to produce 2 moles of H2O. I don't know if the question implies that, so it could also be that 7 mol H2 makes 7 mol H2O. Sorry for the confusion, I hope it helps a bit