Explaining the concept:
Molar mass of an element is the mass of 6.022*10²³ atoms of that element
a group of 6.022*10²³ atoms or molecules is called 1 mole of that specific element or compound
this number is so large that even though atoms are very small, 6.022*10²³ of them weigh several grams depending on the element the atom belongs to.
Atoms with a larger atomic number have more protons and neutrons than atoms with small atomic number; more protons and neutrons means that they have more matter and hence, more mass
that's why elements with a large atomic number are heavier
for example:
the molar mass (mass of 1 mole or 6.022*10²³ atoms) of hydrogen is approximately '1' grams per mole
whereas,
the molar mass of Carbon (C), (which has only 5 electrons more than hydrogen) is 12 grams per mole
Solving the question:
Finding the number of Moles:
We are given 5.4 grams of Boron (B)
the molar mass of boron is 11 grams per mole
To calculate the number of moles of an element, we divide the given mass by the molar mass
Number of moles = Given mass / Molar mass
Number of moles = 5.4 / 11
Number of moles = 0.5 (approx)
Finding the number of Atoms:
Now that we have the number of moles, we can easily find the number of atoms by dividing it by 6.022*10²³ (since a group of 6.022*10²³ atoms is 1 mole)
Number of Atoms = Number of moles * 6.022*10²³
Number of atoms = 0.5 * 6.022*10²³
Number of atoms = 3.011 * 10²³