Question 828932
Starter amount of soda solution is 500*1.08 grams, 540 grams of 8% m/m soda.


You want to add some x amount of 62.9% m/m WATER, which is 37.1% SODA, to the 540 grams to get 15% m/m soda.  To this extent, we are no longer interested in volumes.


I said this, which was WRONG------------------------------------
{{{(540*8+x*62.9)/(x+540)=15}}}
Solve for x, the grams of the "Crystal Soda" 62.9% m/m to add.
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I should have said this, much more correct---------------------
{{{(540*8+37.1*x)/(x+540)=15}}}
Solve for x, the grams of "Crystal Soda", 62.9% m/m water, 37.1% SODA
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One would imagine that Crystal Soda is the name of a concentrated carbonated drink.  On the other hand, 8% sodium hydroxide has a density very close to what you gave.  62.9%, concentration of the water, 37.1% m/m of SODA.