The percentage mass concentration of a substance is calculated by dividing the mass of the substance by the total mass of the solution, multiply by 100%, C = (m in-va / M solution) * 100%. It does not depend on the molar mass of the substance.
In our case, 5% = (m NaCl / 80) * 100%, or 0.05 = m NaCl / 80 means, m NaCl = 80 * 0.05 = 4 g.
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