What are the molecular weights of the following compounds? NaOH, Mn2Se7, MgCl2, H3PO4.
The Correct Answer and Explanation is:
Here are the molecular weights (also known as molar masses) of the given compounds:
- NaOH (Sodium hydroxide)
Molar mass = 22.99 (Na) + 15.999 (O) + 1.008 (H)
= 39.997 g/mol - Mn₂Se₇ (Dimanganese heptaselenide)
Molar mass = 2 × 54.938 (Mn) + 7 × 78.96 (Se)
= 109.876 + 552.72
= 662.596 g/mol
Rounded: 662.60 g/mol - MgCl₂ (Magnesium chloride)
Molar mass = 24.305 (Mg) + 2 × 35.45 (Cl)
= 24.305 + 70.90
= 95.205 g/mol - H₃PO₄ (Phosphoric acid)
Molar mass = 3 × 1.008 (H) + 30.974 (P) + 4 × 15.999 (O)
= 3.024 + 30.974 + 63.996
= 97.994 g/mol
Rounded: 97.99 g/mol
Explanation
Molecular weight, or molar mass, is the mass of one mole of a chemical compound, measured in grams per mole (g/mol). To determine this, you sum the atomic masses of all the atoms in a molecule, using values from the periodic table. Let us examine each compound individually.
Sodium hydroxide (NaOH) contains one sodium (Na), one oxygen (O), and one hydrogen (H) atom. By adding their atomic masses, we find the total molar mass is approximately 40 g/mol. This compound is commonly used in chemistry as a strong base.
Dimanganese heptaselenide (Mn₂Se₇) contains two manganese atoms and seven selenium atoms. Manganese has a relatively high atomic mass, and selenium is even heavier. The large number of selenium atoms contributes significantly to the overall molar mass, which is about 662.60 g/mol. This is a rare inorganic compound studied in advanced chemistry.
Magnesium chloride (MgCl₂) is a salt made of one magnesium ion and two chloride ions. Magnesium is a lighter metal, but the two chlorine atoms contribute a fair amount of mass. Its molar mass is around 95.21 g/mol. This salt is commonly found in seawater and used in medicine and industry.
Phosphoric acid (H₃PO₄) contains three hydrogen atoms, one phosphorus atom, and four oxygen atoms. The high number of oxygen atoms increases its molar mass, giving a total around 97.99 g/mol. It is widely used in fertilizers and soft drinks.
Knowing molar masses is important in chemistry for converting between grams and moles, which is essential for stoichiometry and quantitative chemical analysis.
