1. How many moles is 12.5 g of magnesium hydroxide?
When we want to convert from grams to moles of moles to grams we have to use the molar mass of that compound. So the first step to solve our problem is to determine the molar mass of magnesium hydroxide. The formula of magnesium hydroxide is Mg(OH)₂ and to find its molar mass we need the atomic masses of its elements.
Mg: 24.31 amu O: 16.00 amu H: 1.01 amu
One molecule of Mg(OH)₂ has 1 atom of Mg, 2 atoms of O and two atoms of H. The molar mass of it:|
molar mass of Mg(OH)₂ = 1 * molar mass of Mg + 2 * molar mass of O + 2 * molar mass of H
molar mass of Mg(OH)₂ = 1 * 24.31 + 2 * 16.00 + 2 * 1.01
molar mass of Mg(OH)₂ = 58.33 g/mol
Now we can find the number of moles of Mg(OH)₂ that we have in 12.5 g of it.
number of moles = mass of sample/molar mass
number of moles of Mg(OH)₂ = 12.5 g / (58.33 g/mol)
number of moles of Mg(OH)₂ = 0.214 moles
Answer: there are 0.214 moles of Mg(OH)₂ in 12.5 g of it.
2. How many moles is 1.46 g of hydrogen gas (H₂)?
atomic mass of H = 1.01 amu
molar mass of H₂ = 2 * 1.01 g/mol
molar mass of H₂ = 2.02 g/mol
number of moles = mass of sample/molar mass
number of moles of H₂ = 1.46 g / 2.02 g/mol
number of moles of H₂ = 0.723 moles
Answer: there are 0.723 moles of H₂ in 1.46 g of it.
3. How many grams are in 4.3 moles of ammonium chloride?
The formula of ammonium chloride is NH₄Cl. The atomic mass of its elements is:
atomic mass of N: 14.01 amu
atomic mass of H: 1.01 amu
atomic mass of Cl: 35.45 amu
molar mass of NH₄Cl = 1 * 14.01 + 4 * 1.01 + 1 * 35.45
molar mass of NH₄Cl = 53.5 g/mol
In this case we have to go from grams to moles. We will use a formula similar to the one that we used before.
number of moles = mass of sample/molar mass
mass of sample = number of moles * molar mass
mass of NH₄Cl = 4.3 moles * 53.5 g/mol
mass of NH₄Cl = 230 g
Answer: there are 230 g of NH₄Cl in 4.3 moles of it.