Answer:
180 mg
Explanation:
For a first-order reaction, we can calculate the amount of aspirine (A) at a certain time (t) using the following expression.

where,
k: rate constant
A₀: initial amount
If we know the half-life (
) we can calculate the rate constant.

When t = 4 h and A₀ = 400 mg, A is:

mass (m) = ? , volume = 18.0 ml , density = 1.42 g/ml .

I hope I helped you^_^
Balanced equation:
Ca(OH)2+H2SO4=CaSO4+2 H20<span />
A=mass number (protons + neutrons)
Z=atomic number (protons)
N=number of neutrons.
A=Z+N
Data:
A=24
N=13
Therefore:
24=Z+13
Z=24-13=11
Answer: The atomic number of this atom is 11. (The element is sodium)