### Video Transcript

How many grams of HCl are present in 500 milliliters of a two-molar HCl solution? (A) One, (B) 18.3, (C) 35.5, (D) 36.5, (E) 73.

In this question, we need to calculate the mass of HCl or hydrochloric acid. And we’ve been given a volume and a concentration. Recall that molarity is defined as moles of a substance per liter of solution. So, to solve this problem, the first thing we’ll do is use the volume and the concentration that we’ve been given to find the amount of our substance in moles. From there, we should be able to calculate the mass of HCl in grams.

So, first, let’s calculate the amount of HCl in our solution in moles, which we can do by multiplying our concentration by the volume of our solution. But the volume that we’ve been given is in units of milliliters, and concentration is in units of liters. So, we’re going to need to convert our volume from milliliters to liters so that the units will cancel. We can do this by dividing by 1000 since there are 1000 milliliters in a liter.

We could also use powers of 10 to convert this. Since one milliliter is defined as 10 to the minus three liters. Either way, both approaches are equivalent. If we multiply two by 500, we’ll get 1000. And then, we have to divide by 1000. So, we just end up with one mole. Now, we’re in moles of HCl. And we need the mass of HCl in grams. To do this, we’ll need to calculate the molar mass of HCl, which we can do by summing the molar masses found on the periodic table.

We’ll use rounded molar masses here since for the most part our answers are nice round numbers. Taking one gram per mole as the molar mass of hydrogen and 35.5 grams per mole as the molar mass of chlorine, the molar mass of hydrochloric acid is 36.5 grams per mole. Now, we can find the mass of HCl by multiplying the amount of HCl in moles by its molar mass, which gives us 36.5 grams of HCl, which matches answer choice (D).

So, there are 36.5 grams of HCl present in 500 milliliters of a two-molar HCl solution.