7. how many formula units are equal to a 0.25 g sample of chromium (iii) sulfate, cr₂(so₄)₃?

Explanation:

Step1: Calculate molar mass of $Cr_2(SO_4)_3$

The molar mass of $Cr$ is approximately $52\ g/mol$, $S$ is approximately $32\ g/mol$, and $O$ is approximately $16\ g/mol$. For $Cr_2(SO_4)_3$, molar mass $M=2\times52 + 3\times(32+4\times16)= 392\ g/mol$.

Step2: Calculate number of moles

Use the formula $n=\frac{m}{M}$, where $m = 0.25\ g$ and $M = 392\ g/mol$. So $n=\frac{0.25}{392}\ mol$.

Step3: Calculate number of formula - units

Use Avogadro's number $N_A = 6.022\times 10^{23}\ formula - units/mol$. The number of formula - units $N=n\times N_A=\frac{0.25}{392}\times6.022\times 10^{23}$. $N=\frac{0.25\times6.022\times 10^{23}}{392}\approx3.84\times 10^{20}$ formula - units.

Answer:

$3.84\times 10^{20}$ formula - units