Answer: There are
atoms of hydrogen are present in 40g of urea,
.
Explanation:
Given: Mass of urea = 40 g
Number of moles is the mass of substance divided by its molar mass.
First, moles of urea (molar mass = 60 g/mol) are calculated as follows.

According to the mole concept, 1 mole of every substance contains
atoms.
So, the number of atoms present in 0.67 moles are as follows.

In a molecule of urea there are 4 hydrogen atoms. Hence, number of hydrogen atoms present in 40 g of urea is as follows.

Thus, we can conclude that there are
atoms of hydrogen are present in 40g of urea,
.
They will have different compounds
Answer:
The coefficient for Al(s) is 2.
Explanation:
__ Al(s) __ O2(g) --> __ Al2O3(s)
__ Al(s) __ O2(g) --> 1 Al2O3(s)
1 Al(s) __ O2(g) --> 1 Al2O3(s)
1 Al(s) + 1.5 O2(g) --> 1 Al2O3(s) [Use a fraction to make it balance]
2 Al(s) + 3 O2(g) --> 2 Al2O3(s) [Convert the 1.5 fraction to a whole number by multiplying all coefficients by 2]