How can you prepare percent 3.5 wt NaCl?

Weigh 3g Salt into a 100ml container and bring to volume with deionized water. Mix well. This will be a 3% standard. Other standards are made in the same way (ex.: 2g in 100ml water = 2.00% or 2.5g in 100 ml = 2.50%). I use this in my lab when I make our standard. Hope your not getting this information to late!