Question

-- I can calculate the formula mass but get confused with the various conversions between different types of units. I have the correct answers but I am trying to learn how to work through the problems to get the correct answer.

10) A sample of talc, Mg3Si4O10(OH)2, has a mass of 0.664 g. Calculate the number of mmoles of Mg3Si4O10(OH)2.

11) A sample of gypsum, CaSO4•2H2O, has a mass of 22 g. Calculate the number of moles of CaSO4•2H2O.

12) A sample of potassium dichromate, K2Cr2O7, has a mass of 578 Gg. Calculate the number of moles of K2Cr2O7.

13) A pure sample contains 660 moles of potassium dichromate, K2Cr2O7. What is the mass of the sample?

14) A pure sample contains 3.0 x 10-8 moles of gypsum, CaSO4•2H2O. What is the mass of the sample in units of ng?

15) A pure sample contains 8.8 x 10-2 moles of talc, Mg3Si4O10(OH)2. What is the mass of the sample?

16) A pure sample contains 1.5 moles of ammonium carbonate, (NH4)2CO3. What is the mass of the sample?

The answers are

10) 1.75 mmoles Mg3Si4O10(OH)2

11) 0.13 moles CaSO4•2H2O

12) 1.96 x 109 mol K2Cr2O7

13) 1.94 x 105 g K2Cr2O7

14) 5165 ng CaSO4•2H2O

15) 33.4 g Mg3Si4O10(OH)2

16) 144.1 g (NH4)2CO3

Answer 1

10.) molar mass of Mg₃Si₄O₁₀(OH)₂ = 379.2657 gm/mol that mean 1 mole of Mg₃Si₄O₁₀(OH)₂ = 379.2657 gm

no. of mole = mass/molar mass

0.664 gm of  Mg₃Si₄O₁₀(OH)₂ = 0.664 / 379.2657 = 0.00175 mole

Mg₃Si₄O₁₀(OH)₂ =  0.00175 mole = 1.75 mmole

11) molar mass of CaSO₄. 2H₂O = 172.1712 gm/mole that mean 1 mole of CaSO₄. 2H₂O = 172.1712 gm

no. of mole = mass / molar mass

no, of mole of CaSO₄. 2H₂O = 22/172.1712 = 0.12777979 mole = 0.13 mole

no, of mole of CaSO₄. 2H₂O = 0.13 mole

12) 578 Gg = 5.78 10¹¹ gm

molar mass of K₂Cr₂O₇ = 294.185 gm/mole that mean 1 mole of K₂Cr₂O₇ = 294.185 gm

no. of mole = mass / molar mass = 5.78 10¹¹ /294.185 = 1.96 10⁹ mole

13) molar mass of K₂Cr₂O₇ = 294.185 gm/mole that mean 1 mole of K₂Cr₂O₇ = 294.185 gm

then mass of 660 mole of K₂Cr₂O₇ = 294.185 660 = 1.94   10⁵ gm