Question (1):
One mole of any element contains Avogadro's number of atoms. To know the number of moles in 3.025 * 10^28 atoms, all we have to do is cross multiplication as follows:
1 mole ................> 6.022 * 10^23 atoms
?? moles ............> 3.025 * 10^28 atoms
number of moles = (3.025 * 10^28 atoms * 1) / (6.022 * 10^23)
number of moles = 5.02 X 10^4 mole NaCl
Question (2):
First, we will get the number of moles in 10.5 grams of Ca(OH)2
From the periodic table:
mass of Ca = 40 grams
mass of H = 1 grams
mass of O = 16 grams
Molar mass of Ca(OH)2 = 40 + 2(16+1) = 74 grams
number of moles = mass / molar mass = 10.5 / 74 = 0.14189 moles
Now, one mole of any element contains Avogadro's number of particles. To know the number of particles in 0.14189 moles, all we have to is cross multiplication as follows:
1 mole ...............> 6.022 * 10^23 particles
0.14189 moles ...> ?? particles
number of particles = (0.14189*6.022*10^23) / 1
number of particles = 8.54 * 10^22 particles of Ca(OH)2
Question (3):
One mole of any substance contains Avogadro's number of atoms. Therefore, we will use cross multiplication to get the number of moles containing 8.95 * 10^29 atoms as follows:
1 mole ............> 6.022 * 10^23 atoms
?? moles .........> 8.95 * 10^29 atoms
number of moles = (8.95 * 10^29 * 1) / (6.022 * 10^23) = 1.486 * 10^6 moles
Now, we will get the molar mass of MgBr2. From the periodic table:
mass of Mg = 24.3 grams
mass of Br = 79.9 grams
Therefore,
molar mass of MgBr2 = 24.3 + 2(79.9) = 184.1 grams
Finally, we will get the mass of MgBr2 as follows:
number of moles = mass / molar mass
mass = number of moles * molar mass
mass = 1.486 * 10^6 * 184.1
mass = 2.74 * 10^8 grams MgBr2
Question (4):
First, we will get the number of moles in 50 grams LiBr.
From the periodic table:
mass of Li = 6.9 grams
mass of Br = 79.9 grams
Therefore,
molar mass of LiBr = 6.9 + 79.9 = 86.8 grams
number of moles = mass / molar mass = 50 / 86.8 = 0.576 moles
Now, one mole of any substance contains Avogadro's number of atoms. This means that we can simply use cross multiplication to get the number of atoms in 0.576 moles as follows:
1 mole .................> 6.022 * 10^23 atoms
0.576 moles ........> ?? atoms
number of atoms = (0.576*6.022*10^23) / 1
number of atoms = 3.45 * 10^23 atoms LiBr
Hope this helps :)
