Video Transcript
What is the mass concentration of hydrogen chloride in 2.0-molar hydrochloric acid. Give your answer to the nearest whole number.
The molar mass of hydrogen is one gram per mole and the molar mass of chlorine is 35.5 grams per mole. Hydrochloric acid is an aqueous solution of hydrogen chloride. We are told that the concentration of the hydrochloric acid is 2.0 molar. This is equivalent to 2.0 moles per liter. This is the molar concentration, which is the number of moles of solute per liter of solution. A solute is the minor component of a solution. It is the substance that is described as dissolving. In this case, the solute is the hydrogen chloride. A solution is a mixture in which one or more solutes are distributed uniformly within the solvent. So in this case, the solution is hydrochloric acid. And as hydrochloric acid is an aqueous solution of hydrogen chloride, the solvent would be water.
The equation for molar concentration can be expressed as 𝑐 equals 𝑛 divided by 𝑣, where 𝑛 is the number of moles of solute, 𝑣 is the volume of solution in liters, and 𝑐 is the molar concentration in moles per liter. We have been given the molar concentration in the question. It is 2.0 moles per liter. We can express this in the form 𝑐 equals 𝑛 divided by 𝑣 as 𝑐 equals two moles divided by one liter. But the question doesn’t ask about the molar concentration. It asks about the mass concentration.
The mass concentration is the mass of the solute per liter of solution. We still need to consider the solute hydrogen chloride and the solution hydrochloric acid, and the equation is almost the same. But instead of using the number of moles of solute, we use the mass of the solute in grams. 𝑣 still represents the volume of solution in liters, but 𝑐 now represents the mass concentration in grams per liter. As the molar concentration and mass concentration equations are so similar, we can use the format for the molar concentration to help us find the mass concentration. We just need to replace the two moles of hydrogen chloride with the mass that’s equivalent to two moles of hydrogen chloride.
To help us do this, we can use this equation which tells us that the number of moles is equivalent to the mass divided by the molar mass. We want to find the mass. So we need to make the mass the subject in this equation. If we multiply both sides of the equation with the molar mass, the molar mass on the right side of the equation will cancel, leaving us with the mass equals the number of moles multiplied by the molar mass. We already know that we’re calculating the mass for two moles of hydrogen chloride. We need to multiply the number of moles of hydrogen chloride by the molar mass of hydrogen chloride. We can use the molar masses in the question to help us do this.
Hydrogen chloride has the formula HCl, so we need to multiply the molar mass of hydrogen, which is one gram per mole, by the number of equivalents of hydrogen in hydrogen chloride, which is one. We then need to add the molar mass of chlorine, which is 35.5 gram per mole, and multiply it by the number of equivalents of chlorine in hydrogen chloride, which is also one. So we need to multiply two moles by 36.5 gram per mole. This gives a mass of 73 grams. We can now put this value into the mass concentration equation. So the mass concentration is 73 grams per one liter. We can rewrite this as 73 grams per liter.
So the answer to the question “What is the mass concentration of hydrogen chloride in 2.0-molar hydrochloric acid?” is 73 grams per liter.
