Question

a chemist adds 1.25 l of a 0.16 mol/l potassium dichromate (k₂cr₂o₇) solution to a reaction flask. calculate the mass in grams of potassium dichromate the chemist has added to the flask. be sure your answer has the correct number of significant digits.

Explanation:

Step1: Calculate moles of $K_2Cr_2O_7$

Use the formula $n = cV$, where $n$ is the number of moles, $c$ is the concentration and $V$ is the volume.
$n=0.16\ mol/L\times1.25\ L = 0.2\ mol$

Step2: Calculate molar - mass of $K_2Cr_2O_7$

The molar - mass of $K$ is $39.1\ g/mol$, $Cr$ is $52.0\ g/mol$ and $O$ is $16.0\ g/mol$.
$M=(2\times39.1 + 2\times52.0+7\times16.0)\ g/mol=(78.2 + 104.0 + 112.0)\ g/mol = 294.2\ g/mol$

Step3: Calculate mass of $K_2Cr_2O_7$

Use the formula $m = nM$, where $m$ is the mass, $n$ is the number of moles and $M$ is the molar - mass.
$m = 0.2\ mol\times294.2\ g/mol=58.84\ g$
Rounding to 2 significant digits (since 0.16 has 2 significant digits), $m = 59\ g$

Answer:

$59\ g$