Explanation:
First, I will assume you are asking the procedure to get mass of a substance, after we initiate a chemical reaction. If this is the case, then, let's explain it with an example:
Let's suppose we have the following reaction:
Zn + 2HCl -------> ZnCl₂ + H₂
And we part with 4.55 g of Zinc (Zn). We want to know the mass produced of ZnCl₂.
This is a simple reaction, where we only do conservation of mass. So, to get the mass of ZnCl2, we need to begin with the Zinc first, because we start with that given mass.
the mass of Zinc is 4.55 g. Chemicals reactions are better worked in moles rather than mass, because coefficients in the chemical reaction are in moles. So, in the case of the reaction (Which is already balanced):
Zn + 2HCl -------> ZnCl₂ + H₂
The number 2 that it's behind the HCl represents the theorical moles of that substance, in this case, we need 2 moles of HCl to react with only 1 mole of Zinc. It's 1 mole of Zinc because there are no numbers there, so, we always assume there's a 1 in there.
Now that we know this, let's calculate the moles of Zinc. To do that, we need the molar mass of Zinc. This can be obtained in the exercise or look the periodic table, and search for the Zinc. In this case, the molar mass of Zinc is 65.38 g/mol. So to get the moles:
n = m/MM
Replacing the data:
n = 4.55 g / 65.38 g/mol = 0.0695 moles
These are the moles of Zn reacting in the chemical reaction. Now, we should do some stechiometry. In this case, we can see that, 1 mole of zinc (In the reaction) produces 1 mole of ZnCl2, therefore the moles of Zn obtained, will be the same moles produced of ZnCl2, thus:
moles ZnCl2 = moles Zn = 0.0695 moles
Finally, with the molar mass of ZnCl2, we can calculate the mass. the molar mass of ZnCl2 is 132.29 g/mol so:
m = n * MM
Replacing:
m = 0.0695 * 132.29
<em>m = 9.19 g of ZnCl2</em>
<em>And this is the simple way to obtain the mass of a substance in a reaction</em>
