1. The problem statement, all variables and given/known data A gas cylinder contains 4.00 x 104 cm3 of hydrogen at a pressure of 2.50x107 Pa and a temperature of 290 K. The cylinder is to be used to fill balloons. Each balloon, when filled, contains 7.24x103 cm3 of hydrogen at a pressure of 1.85x105 Pa and a temperature of 290 K. Calculate, assuming that the hydrogen obeys the equation in (a), (i) the total amount of hydrogen in the cylinder, (ii) the number of balloons that can be filled from the cylinder. 2. Relevant equations pV=nRT If there are any others I don't know about them. 3. The attempt at a solution (i) You get the value for n from the ideal gas equation. Simple. 415. [rounded to three s.f.] (ii) So what I did here was that I found the value of n for one balloon [again, the values are given in the question]. I got 0.55, and then I divided the total number of moles, 415, by 0.55. I get 746. The problem? The book mark scheme says it's supposed to be 741. What am I doing wrong in that second part? Please help.