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How would my sample be affected if the sample was not totally dry? What would happen to the mass percent composition? I believe that it will increase but someone argues that due to ratios, it will decrease.

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  • $\begingroup$ If sample is not fully dry, obviously this would increase the mass or weight of sample. $\endgroup$ – Khan Aug 28 '16 at 4:14
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The IUPAC Compendium of Chemical Terminology defines the mass fraction $w$ as the

mass of a constituent divided by the total mass of all constituents in the mixture

$$w_i = \frac{m_i}{\sum\limits_{i=1}^{N}m_i} \tag{1}$$

This means that $w_i$ can adopt values between 0 ( = 0 %) when the compound in question is not contained in the sample), and 1 ( = 100 %) when the sample only consists of the compound.

If your sample isn't totally dry, the mass of water adds to the total mass of the sample.

As a result, the denominator in $(1)$ will become larger and the mass fraction $w_i$ for any component (except the water) will decrease.

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