About the measuring thing: you can put anything on your scale (I used a glass, to make sure it wouldn't get all over the place), make sure it's turned off, turn on and add SN, it won't measure the "container". If you want to be extra precise, you can buy a jewelry scale. But to be honest, you don't have to be that precise. Same with water for the solution, you can use the scale or a measuring cup.
About the SN itself: Nothing will happen to it if its unopened, keep it in room temperature in a cupboard or something like that, away from sunlight. (Light can make it oxidize) I opened mine 4 months before attempting and it still worked. Just make sure it's air tight and it should be fine + re-do the test before taking it, just to make sure.