First we get the mass of NH3 (ammonia) in the concentrated NH3 solution, by using the relationship between density, mass and volume:
d = m/V
0.88 = m/28.2
m = 24.816 g
Since ammonia is only 28% of this mass,
m,NH3 = 24.816 * 0.28
m,NH3 = 6.9485 g NH3
Then we change this to moles. To change this to moles, we divide this mass by the molar mass of NH3. To get the molar mass, just get a periodic table and add the individual masses of the elements in the chemical formula:
Molar Mass of NH3: 1*14 + 3*1 = 17 g/mol
n,NH3 (moles of NH3) = 6.9485 / 17
n,NH3 = 0.4087 mol NH3
Finally, recall that molarity is just moles of solute per liter of solution,
M = n/V
Therefore, (500 mL = 0.5 L)
M = 0.4087 mol NH3 / 0.5 L
M = 0.817 M
Hope this helps~ :3
Calculate the molarity of a solution made by adding 28.2 mL of concentrated ammonia (28.0 % by mass, density 0.880 g/mL) to some water in a volumetric flask, then adding water to the mark to make exactly 500 mL of solution. (It is important to add concentrated acid or base to water, rather than the other way, to minimize splashing and maximize safety.)
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