Mass fraction (chemistry)

In <a href="/facts/Chemistry/SrNqkGVm">chemistry</a>, the mass fraction of a substance within a mixture is the ratio 
 
 
 
 
 w
 
 i
 
 
 
 
 {\displaystyle w_{i}}
 
 (alternatively denoted 
 
 
 
 
 Y
 
 i
 
 
 
 
 {\displaystyle Y_{i}}
 
) of the mass 
 
 
 
 
 m
 
 i
 
 
 
 
 {\displaystyle m_{i}}
 
 of that substance to the total mass 
 
 
 
 
 m
 
 tot
 
 
 
 
 {\displaystyle m_{\text{tot}}}
 
 of the mixture. Expressed as a formula, the mass fraction is:

w
          
            i
          
        
        =
        
          
            
              m
              
                i
              
            
            
              m
              
                tot
              
            
          
        
        .
      
    
    {\displaystyle w_{i}={\frac {m_{i}}{m_{\text{tot}}}}.}

Because the individual masses of the ingredients of a mixture sum to 
 
 
 
 
 m
 
 tot
 
 
 
 
 {\displaystyle m_{\text{tot}}}
 
, their mass fractions sum to unity:

∑
          
            i
            =
            1
          
          
            n
          
        
        
          w
          
            i
          
        
        =
        1.
      
    
    {\displaystyle \sum _{i=1}^{n}w_{i}=1.}

Mass fraction can also be expressed, with a <a href="/facts/Denominator/ghnQE9B3">denominator</a> of 100, as percentage by mass (in commercial contexts often called percentage by weight, abbreviated wt.% or % w/w; see <a href="/facts/Mass_versus_weight/gNvnevJE">mass versus weight</a>). It is one way of expressing the composition of a mixture in a <a href="/facts/Dimensionless_quantity/TINrpQIq">dimensionless size</a>; <a href="/facts/Mole_fraction/nW4EepmJ">mole fraction</a> (percentage by <a href="/facts/Mole_(unit)/kCMFwCc0">moles</a>, mol%) and <a href="/facts/Volume_fraction/nj2akBNM">volume fraction</a> (<a href="/facts/Volume_percent/nj2akBNM">percentage by volume</a>, vol%) are others.
When the prevalences of interest are those of individual <a href="/facts/Chemical_elements/0jducgWD">chemical elements</a>, rather than of compounds or other substances, the term mass fraction can also refer to the ratio of the mass of an element to the total mass of a sample. In these contexts an alternative term is mass percent composition. The mass fraction of an element in a compound can be calculated from the compound's <a href="/facts/Empirical_formula/xd0vSIlm">empirical formula</a> or its <a href="/facts/Chemical_formula/hghUtthl">chemical formula</a>.

Mass fraction (chemistry) open-in-new

Mass fraction (chemistry)