Question Video: Formula Mass and Mole Concept: Calculation of Molar Mass | Nagwa Question Video: Formula Mass and Mole Concept: Calculation of Molar Mass | Nagwa

# Question Video: Formula Mass and Mole Concept: Calculation of Molar Mass Chemistry • First Year of Secondary School

Which of these substances has the largest formula mass? [Ca =12, H = 1, O = 16, Na = 23, Br = 80, Cl = 35.5, F = 19, N = 14]. [A] The anesthetic halothane (C₂HBrClF₃) [B] The herbicide paraquat (C₁₂H₁₄N₂Cl₂) [C] Caffeine (C₈H₁₀N₄O₂) [D] Urea (CO(NH₂)₂) [E] A typical soap (C₁₇H₃₅CO₂Na)

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### Video Transcript

Which of these substances has the largest formula mass? The relative atomic mass of carbon is 12, hydrogen is one, oxygen 16, sodium 23, bromine 80, chlorine 35.5, fluorine 19, and nitrogen 14. (A) The anesthetic halothane, C2HBrClF3; (B) the herbicide paraquat, C12H14N2Cl2; (C) caffeine, C8H10N4O2; (D) urea, CO(NH2)2; (E) a typical soap, C17H35CO2Na.

The formula mass is the average mass of one formula unit of a compound. We can calculate the formula masses of each compound we were given using the relative atomic masses given in the problem. The relative atomic mass is the average mass of one atom of an element relative to the mass of carbon-12, which is defined to be 12. In reality, the mass of the carbon-12 atom is about 1.99 times 10 to the negative 23 grams. But that’s not a very convenient number to work with in calculations, which is why we use relative atomic masses.

Let’s calculate the formula masses of each compound given in the answer choices, starting with the anesthetic halothane. We can determine which elements are present in halothane by looking at the chemical formula. Halothane contains the elements carbon, hydrogen, bromine, chlorine, and fluorine. The relative atomic masses of each element were listed in the problem. We can look at the subscripts in the chemical formula to determine how many atoms of each element are in one formula unit. There are two carbon atoms, one hydrogen atom, one bromine atom, one chlorine atom, and three fluorine atoms in one formula unit of halothane.

To calculate the formula mass, we just multiply the mass of each element by the number of atoms in one formula unit and add the results. Doing that calculation gives us 197.5. So, let’s take note of that and move on to the next molecule, the herbicide paraquat.

Paraquat contains the elements carbon, hydrogen, nitrogen, and chlorine. We’ll note that and the relative atomic masses of each element. There are 12 carbon atoms, 14 hydrogen atoms, two nitrogen atoms, and two chlorine atoms in one formula unit of paraquat. If we multiply and add all those values, we’ll find the formula mass of paraquat is 257. So, let’s note that formula mass as well and move on to caffeine.

We can peek at the chemical formula to figure out which elements are in caffeine. Then, we’ll note the relative atomic masses of each. Now, we’ll multiply those atomic masses by the number of atoms of each element in caffeine. That calculation gives us 194 for the formula mass.

Now we can calculate the mass of urea, answer choice (D). Urea contains carbon, oxygen, nitrogen, and hydrogen. Let’s note the number of atoms indicated by the chemical formula in urea. Remember the subscript two outside the parentheses indicates that there are two units of NH2 in urea. So, there are a total of two nitrogen and four hydrogen atoms in one formula unit of urea. Multiplying then adding all those together gives us 60 as the formula mass of urea.

Now, we can move on to the final substance we were given, a typical soap. A typical soap contains carbon, hydrogen, oxygen, and sodium. Looking at the number of each atom in a formula unit, note that carbon appears in two places in the formula. So, there are a total of 18 carbon atoms in a formula unit. If we do the math, we’ll find the formula mass of a typical soap is 306.

Now, we’ve calculated the formula masses of all the substances given in the problem. Comparing the formula masses that we calculated, we can see that a typical soap is the substance that has the largest formula mass. So, answer choice (E) is the correct answer.

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