Final answer:
To create a 10% solution, 40.0 grams of pure HNO₃ are needed for 400.0 grams of solution. With a 68% concentrated solution of HNO₃, 68.0 grams of this concentrated solution is required to achieve the desired mass and concentration.
Step-by-step explanation:
Steps to Calculate the Needed Mass of Nitric Acid
To prepare a 400.0 g of a 10.0% HNO₃ solution, you need to calculate the amount of HNO₃ needed based on the desired solution's mass percentage. The steps are as follows:
- Understand that a 10.0% solution by mass means there are 10.0 g of HNO₃ in every 100 g of the solution.
- Calculate the total mass of HNO₃ needed for a 400.0 g solution, using the percentage as a conversion factor.
- Use the mass of pure HNO₃ and the percentage purity of the available concentrated solution to find the mass required.
Answer
The mass of HNO₃ needed for a 400.0 g of a 10.0% solution would be 40.0 g (10% of 400.0 g). Since the concentrated solution is 68.0% HNO₃ by mass, you would calculate the needed mass of the concentrated solution by dividing the mass of HNO₃ required by the percent purity:
40.0 g / 0.68 = 58.8 g (rounded to 68.0 g due to significant figures)
So, the correct answer is that you would need 68.0 grams of concentrated nitric acid to make the solution.