How to Calculate Formula Unit Mass: Step-by-Step Guide
The mass of an atom/molecule can be determined by mass spectroscopy. The macroscopic objects can be calculated in units of kilograms, tons, etc., whereas the microscopic particles can be calculated in atomic mass units. Hydrogen is taken as the reference mass to determine the mass of multi-electron atoms because of having a unity mass. Later, carbon having atomic mass 12 is used as a reference due to the abundance of the atom in the environment. The carbon having atomic mass 12 is used because carbon having atomic mass 13 and 14 are radioactive atoms.
The molecular mass unit can be defined as the molecule's mass, formulated from the molecular formula. The formula unit mass is the mass of the chemical formula of the molecule, which shows the number of atoms present in the molecule. The atomic mass is used to formulate the formula mass of the chemical formula.
What is a Formula Mass Unit?
The formula mass unit is the mass of the chemical formula of the compound present. The formula mass unit can be formulated from the addition of the number of moles of the atom present in the compound. The formula mass unit has a unit of amu similar to the molecular mass unit. The calculated mass of the formula mass unit can be the same or different from the molecular mass unit. The formula mass unit does not depend upon the attraction between the molecules. The formula mass unit depends upon the empirical formula of the compound.
Formula Mass Definition
The mass of the atom can be used to calculate the formula mass unit. The formula mass depends upon the number of moles of atoms involved and not on the attractive force involved in forming a dimer or trimer, but the molecular mass unit depends on the attractive force such as forming a dimer or trimer, etc.
A formula mass unit can be defined as the addition of the atomic masses of the atoms involved in the compound. Generally, the formula mass unit is used to formulate the mass of the compounds made up of ions, i.e., ionic compounds.
The unit of the formula mass unit is the atomic mass unit (amu) or um (unified mass). The atomic mass unit can be defined as the mass of the twelfth part of the carbon having atomic mass 12. Earlier, hydrogen was used as the reference to find out the mass of the other multi-electron atoms as they have an atomic mass of unity. But sometimes, carbon has atomic mass 12 to reference atomic mass because of the abundance of the carbon-atom present in the surroundings.
Formula Mass Unit Example
Formula mass units can be formulated by adding the mass of the number of moles of the atom present in the compound. The formula mass unit is based upon the empirical formula of the compound.
Now, we will find the formula mass unit of the ionic compound such as NaCl. The ionic compounds are those which are made up of the ions such as cations and anions in which one ion is positively charged, and the other ion is an anion, respectively.
The ionic compound having cation Na+ and the anion Cl- is involved.
The formula mass unit of the NaCl compound is-
Formula Mass Unit = Number of Moles x Atomic Mass of Na + Number of Moles x Atomic Mass of Cl
Formula mass unit = 1X23 + 1 X 35.5 = 58.5u
The formula mass unit of the NaCl is 58.5u or 58.5 amu.
Interesting Facts
Mass is used to measure the weight of an object.
A formula unit is the chemical formula of an ionic compound that lists the ions in the lowest ratio that equals a neutral electric charge.
The chemical formula is used to describe the elements or molecules that make up a substance.
Key Features of Formula Unit Mass
The unit of the formula mass unit is the atomic mass unit.
The molecular mass unit is the mass of the molecular mass, whereas the formula mass unit is the mass of the chemical formula concluded.
The formula unit mass can be formulated by adding the atomic mass of the number of moles of the atom present in the formula.
Macroscopic particles are those particles which are visible to the naked eye, whereas microscopic particles are those which are not visible to the naked eye.
FAQs on What is Formula Unit Mass?
1. What is formula unit mass, and for which type of compounds is it used?
Formula unit mass is the sum of the atomic masses of all atoms present in one formula unit of a compound. This term is specifically used for substances whose constituent particles are ions, such as ionic compounds. For example, in Sodium Chloride (NaCl), the formula unit mass is calculated by adding the atomic mass of one sodium atom and one chlorine atom.
2. What is the unit used to express formula unit mass?
The unit for formula unit mass is the atomic mass unit, which is abbreviated as 'u' (unified mass). This is the same standard unit used for expressing atomic mass and molecular mass, based on the carbon-12 scale.
3. How do you calculate the formula unit mass of Calcium Chloride (CaCl₂)?
To calculate the formula unit mass of Calcium Chloride (CaCl₂), you sum the atomic masses of all atoms in its formula unit. The atomic mass of Calcium (Ca) is 40 u and the atomic mass of Chlorine (Cl) is 35.5 u. The calculation is as follows:
- Mass of one Ca atom = 1 × 40 u = 40 u
- Mass of two Cl atoms = 2 × 35.5 u = 71 u
Formula Unit Mass of CaCl₂ = 40 u + 71 u = 111 u.
4. What is the key difference between formula unit mass and molecular mass?
The key difference lies in the type of compound they describe:
- Formula Unit Mass: This is used for ionic compounds (like NaCl, MgO) which do not exist as single, discrete molecules but as a three-dimensional crystal lattice of ions. It represents the mass of the simplest whole-number ratio of ions in the compound.
- Molecular Mass: This is used for covalent compounds (like H₂O, CO₂) that exist as distinct, independent molecules. It represents the mass of a single molecule.
5. Why is it incorrect to use the term 'molecular mass' for Sodium Chloride (NaCl)?
It is incorrect because Sodium Chloride (NaCl) is an ionic compound that does not form individual NaCl molecules. Instead, it forms a vast, ordered crystal lattice where each sodium ion (Na⁺) is surrounded by six chloride ions (Cl⁻), and each Cl⁻ is surrounded by six Na⁺ ions. The term 'molecule' implies a discrete, separate entity. Since these do not exist for NaCl, we use the term 'formula unit' to represent the simplest ratio of ions (1:1), and its mass is correctly called the formula unit mass.
6. How is the concept of formula unit mass important for understanding the mole concept?
The concept of formula unit mass is crucial for applying the mole concept to ionic compounds. The mass of a substance in grams that is numerically equal to its formula unit mass is called its gram formula mass. One gram formula mass of any ionic compound contains Avogadro's number (6.022 × 10²³) of formula units. For example, the formula unit mass of NaCl is 58.5 u, so 58.5 g of NaCl is 1 mole and contains 6.022 × 10²³ formula units of NaCl, which is essential for all stoichiometric calculations.
7. Provide some examples of how to find the formula unit mass for common ionic compounds.
Here are a few examples of calculating formula unit mass:
- Zinc Oxide (ZnO):
(Atomic mass of Zn) + (Atomic mass of O)
65 u + 16 u = 81 u - Potassium Carbonate (K₂CO₃):
(2 × Atomic mass of K) + (1 × Atomic mass of C) + (3 × Atomic mass of O)
(2 × 39 u) + (1 × 12 u) + (3 × 16 u) = 78 u + 12 u + 48 u = 138 u
