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PBCHEM
Answer: 1.675 X [tex] 10^{22} [/tex] molecules

Reason:
1 mole of any substance contains Avagadro's number (i.e. 6.022 X  [tex] 10^{23} [/tex] ) of molecules.

 For NO2, molecule weight = 46 g/mol.
 Thus, 1 mol (46 g/mol) of NO2 contains 6.022 x  [tex] 10^{23} [/tex]  molecules.
 Therefore, 1.28 g of NO2 contains 1.675 X  [tex] 10^{22} [/tex]  molecules.
dexteright02

Hello! 

How many molecules of nitrogen dioxide are in 1.28 g of nitrogen dioxide NO2?

Let's first find the molecular mass of nitrogen dioxide, knowing that by Avogadro's Law for each mole of a substance we have 6.02 * 10²³ molecules.

N = 1*14 = 14 amu

O = 2*16 = 32 amu

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molecular mass of nitrogen dioxide = 14 + 32 = 46 g/mol

How many molecules of nitrogen dioxide are in 1.28 g of nitrogen dioxide NO2?

46 g ---------------- 6.02*10²³ molecules

1.28 g -------------  y molecules

[tex]46*y = 1.28*6.02*10^{23}[/tex]

[tex]46y = 7.7056*10^{23}[/tex]

[tex]y = \dfrac{7.7056}{46}*10^{23}[/tex]

[tex]y = 0.167513043*10^{23}[/tex]

[tex]\boxed{\boxed{y \approx 1.675*10^{22}\:molecules\:of\:nitrogen\:dioxide}}\end{array}}\qquad\checkmark[/tex]

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I Hope this helps, greetings ... Dexteright02! =)