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# Chemistry: Calculating the Mass of the Reaction and the Concentration

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For the reaction shown, calculate how many moles of NO 2 form when each amount of reactant completely reacts. 2N 2 O 5 (g)→4NO 2 (g)+O 2 (g)

2.0mol N 2 O 5
6.4mol N 2 O 5
10.0g N 2 O 5
1.50kg N 2 O 5

For each reaction, calculate the mass (in grams) of the product formed when 10.1g of the underlined reactant completely reacts. Assume that there is more than enough of the other reactant.

2K(s)+Cl 2 (g) − − − − − →2KCl(s)
2K(s)+Br 2 (l) − − − − − →2KBr(s)
4Cr(s)+3O 2 (g) − − − − − →2Cr 2 O 3 (s)
2Sr(s) − − − − +O 2 (g)→2SrO(s)

What is the concentration of K + in 0.15 M of K 2 S ?
A scientist wants to make a solution of tribasic sodium phosphate, Na 3 PO 4 , for a laboratory experiment. How many grams of Na 3 PO 4 will be needed to produce 275mL of a solution that has a concentration of Na + ions of 1.40M ?

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The expert calculates the mass of the reaction and the concentration are determined.

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For no.1 a-d see attachment:
2. a.)
2K(s)+Cl 2 (g) − − − − − →2KCl(s)

molar mass of Cl = 35.45 g/mole
molar mass of K = 39.10 g/mole
calculate moles of Cl2: 10.1/2 (moles) = 5.05 g (per mole)/35.45 g
= 0.14245 moles of Cl
mass of K is 0.14245 * 39.10 = 5.57 g.

need twice this amount of K since you have TWO moles of Cl

11.14 g of K + 10.1 g of Cl = total mass of 21.24 g of KCl(s)

2. b.)
2K(s)+Br 2 (l) − − − − − →2KBr(s)

molar mass of Br = 79.9 g/mole
molar mass of K = ...

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