What is the percentage by mass of chromium in sodium dichromate (VI) Na2
Cr2
O7
?​

39.7%

Explanation:

To obtain the percentage composition of chromium (Cr) in Na₂Cr₂O₇, do the following:

Molar mass of Na₂Cr₂O₇ = (23×2) + (52×2) + (16×7)

= 46 + 104 + 112

= 262 g/mol

Mass of Cr in Na₂Cr₂O₇ = 2 × 52 = 104 g

Percentage composition of Cr = mass of Cr / Molar mass of Na₂Cr₂O₇ × 100

= 104 / 262 × 100

= 39.7%

Thus, the percentage composition of chromium (Cr) in Na₂Cr₂O₇ is 39.7%

chemical changes occur when a substance combines with another to form a new substance, called chemical synthesis or, alternatively, chemical decomposition into two or more different substances. these processes are called chemical reactions and, in general, are not reversible except by further chemical reactions. some reactions produce heat and are called exothermic reactions and others may require heat to enable the reaction to occur, which are called endothermic reactions. understanding chemical changes is a major part of the science of chemistry.