Question

question 1:

what is the mass in grams of 1.94x10^24 atoms of selenium?

how manu selenium atoms are there in 31.5 grams of selenium?

question 2:

how many molecules are in 5.87 grams of silicon dioxide?

what is the mass in grams of 6.81x10^22 molecules of silicon dioxide?

Answer 1

6.023 x 10²³ atoms or molecules, that are contained in the amount of substance given by one mole.

Question 1a

No of Selenium atom =  1.94x10²⁴

Molar mass of selenium = 78.96 g/mol

Moles of Selenium = 1.94x10²⁴ / 6.023 x 10²³ = 3.2209 Moles

Mass in grams =  3.2209 Moles x 78.96 g/mol = 254.329 gm

Question 1b

Mass in grams of selenium = 31.5 gm

Moles of Selenium = 31.5 gm / 78.96 gm/mol = 0.3989 Moles

No of selenium atoms =  0.3989 x 6.023 x 10²³ = 2.40 x 10²³ atoms

Question 2a

Mass in grams of silicon dioxide = 5.87 gm

Molar mass of silicon dioxide =  60.08 g/mol

Moles of silicon dioxide = 5.87 gm / 60.08 gm/mol = 0.0977 Moles

No of silicon dioxide molecules =  0.0977 x 6.023 x 10²³ = 5.88 x 10²² atoms  

  

Question 2b

No of silicon dioxide atom =  6.81 x 10²²

Molar mass of silicon dioxide =60.08 g/mol

Moles of silicon dioxide = 6.81 x 10²² / 6.023 x 10²³ = 0.1130 Moles

Mass in grams =  0.1130 Moles x 60.08 g/mol = 6.793 gm