A balloon with a volume of 4.00 L is known to contain 0.200 moles of gas. How many moles of gas remain if some of the gas is released and the new volume is 3.25 L. Assume temperature and pressure remain constant. Please show work, thank you :D.

Astudent pours 10.0 g of salt into a container of water and observes the amount of time it takes for the salt to dissolve. she then repeats the process using the same amounts of salt and water but this time she slowly stirs the mixture while it is dissolving. the student performs the experiment one more time but this time she stirs the mixture rapidly. the dependent variable in this experiment is: time for salt to dissolve speed of stirring amount of water mass of salt