if 255 g of water has 10.0 g of nacl dissolved into initially, how much nacl must be added in order to raise the mass percent of nacl by 10%?

Mass percentage is one way of representing the concentration of an element in a compound or a component in a mixture. To calculate percent by mass, you need to determine two things: the mass of just the element, and the molar mass of the whole compound. We calculate as follows:

.10 g NaCl / g NaCl + Water = ( 10.0 g NaCl + x ) / (10.0 g + 255 g + x )

x = 18.33 g NaCl needed