If you want to gain mass in your arms, you need to be doing the compound lifts - squats, deadlifts, etc. If you're doing exercises for arms, I recommend sticking to the compounds dips and biceps curls and doing variations of them. As far as reps go, 10-12 reps is generally the "mass building" rep range, but everyone is different and responds to different rep ranges - for example, I gain mass with as low as 4-5 reps, but with as high as 15-20 reps (and always lifting as heacy as I can for the set reps).
~Team APPNUT