You wish to make a 0.335 M hydrobromic acid solution from a stock solution of 12.0 M hydrobromic acid.

Question

How much concentrated acid must you add to obtain a total volume of 75.0 mL of the dilute solution?

Answer 1

To solve this problem, first we will need to determine the amount of hydrobromic acid required to create the dilute solution in moles, and then we can find out the volume of the concentrated solution that will have exactly that many moles. Both of these steps use the equation C = n / V, where C is the concentration of a particular solution, n is the number of moles in the solution, and V is the volume of the solution.

First, to find the amount of moles of hydrobromic acid in the dilute solution, we can solve the equation above for n, which gives n = C * V. We are given that the concentration is 0.335 M, or 0.335 mols/L. Therefore, we need to multiply by the volume in liters to get our answer in the correct units. There are 1000 mL in 1 L, so we divide 75.0 mL by 1000 to get the volume in liters, which would be 0.0750 L. Then, we multiply 0.335 mols/L (C in the equation) by 0.075 L (V in the equation) to find that the solution will contain 0.025125 moles of hydrobromic acid.

Now that we know the amount of hydrobromic acid in the solution in moles, we can find out how much of the concentrated solution we need to add to get this number of moles. Solving the equation C = n / V for the volume, we get V = n / C. We know both the concentration and the number of moles, so we can simply plug these in to get our answer: V = 0.025125 mols / (12.0 mols/L) = 0.0020938 L. The problem statement givens have three significant figures, and so we should report our answer to three significant figures. Thus, our answer is 0.00209 L. You may also want to report this in scientific notation (2.09 x 10^-3 L) or in milliliters (2.09 mL) depending on you class's/teacher's preferences.

Answer 2

Hello Alexandria,

You are performing a dilution of HBr going from a concentration of 12M to 0.335M, and you want to end up with a final volume of 75 mL of the dilute solution.

Consider the dilution formula: M₁V₁ = M₂V₂. This basis behind this formula is that the number of moles of the acid before and after the dilution must remain constant. M₁ = the molarity of the stock solution, M₂ = the molarity of the diluted solution, V₂ = the final volume of the diluted solution. In this case, we need to determine V₁, which is the volume of the stock solution used to prepare the diluted sample. With this knowledge, we can plug in our numbers into the equation and we obtain the following: (12 mol/L)*V₁ = (0.335mol/L)*(0.075L).

Keeping in mind that molarity is the moles of a substance in one liter of solution, we will use mol/L instead of M. By doing this, we are reminded that in order to use this equation, we must convert 75 mL into units of liters.

After rearranging the equation and solving for V₁, we find that V₁ = 0.00209L. Finally, we must convert back from liters to mL by multiplying the final answer by 1000. This way we end up with V₁ = 2.09 mL. This means that we need to add 2.09 mL of concentrated acid to obtain 75 mL of 0.335 M HBr solution.