What is the molar mass of $H_3PO_4$?
(Molar mass of $H = 1.0079 g/mol; P = 30.974 g/mol; O = 15.999 g/mol$)
A. $72.98 g/mol$
B. $78.22 g/mol$
C. $88.24 g/mol$
D. $97.99 g/mol
Answer (2)
Calculate the total mass of hydrogen: 3 × 1.0079 = 3.0237 g / m o l .
Calculate the total mass of phosphorus: 1 × 30.974 = 30.974 g / m o l .
Calculate the total mass of oxygen: 4 × 15.999 = 63.996 g / m o l .
Add the masses to find the molar mass of H 3 P O 4 : 3.0237 + 30.974 + 63.996 = 97.9937 g / m o l ≈ 97.99 g / m o l . The final answer is 97.99 g / m o l .
Explanation
Problem Analysis We want to calculate the molar mass of H 3 P O 4 . We are given the molar masses of hydrogen ( H ), phosphorus ( P ), and oxygen ( O ). We need to multiply the molar mass of each element by the number of atoms of that element in the molecule and then add the results.
Calculating Mass of Hydrogen The molar mass of H is 1.0079 g / m o l , and there are 3 hydrogen atoms in the molecule. So, the total mass of hydrogen is 3 × 1.0079 = 3.0237 g / m o l .
Calculating Mass of Phosphorus The molar mass of P is 30.974 g / m o l , and there is 1 phosphorus atom in the molecule. So, the total mass of phosphorus is 1 × 30.974 = 30.974 g / m o l .
Calculating Mass of Oxygen The molar mass of O is 15.999 g / m o l , and there are 4 oxygen atoms in the molecule. So, the total mass of oxygen is 4 × 15.999 = 63.996 g / m o l .
Calculating Total Molar Mass Now, we add the masses of all the elements to find the molar mass of H 3 P O 4 : 3.0237 + 30.974 + 63.996 = 97.9937 g / m o l
Rounding to two decimal places, we get 97.99 g / m o l .
Final Answer The molar mass of H 3 P O 4 is 97.99 g / m o l .
Examples
Understanding molar mass is crucial in chemistry for calculating the amounts of substances needed in reactions. For instance, if you're neutralizing a base with phosphoric acid, knowing the molar mass helps you determine exactly how much acid to use to achieve the desired pH level. This ensures accurate and safe chemical processes, whether in a lab or in industrial applications, preventing over- or under-treatment of materials.
The molar mass of phosphoric acid, H 3 P O 4 , is calculated by adding the molar masses of its constituent elements: hydrogen (3 atoms), phosphorus (1 atom), and oxygen (4 atoms). This summation yields a total of 97.99 g/mol . Therefore, the correct answer is D: 97.99 g / m o l .
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