How would a chemist make up 100g of 10% aqueous solution of HCl (using 12M HCl)

Answer 1

12M HCl is about 37% HCl.
10% x 100 g = 37%x
x = 10*100/37 = about 27 g
Take 27 g 12M HCl and add water to make 100 g.