# Stoichiometric mass to mass conversions?

LOOK at numbers 74 and 75 on the link I have no idea how to do them and I need them for homework I've been trying for the past 20 minutes are so esp 74 please help!!! :O

http://classjump.com/shermanschemistrysite/documen...

Relevance

74.

a.

Steps:

1.Determine the number of moles of 25.0 grams NaCN using its molar mass.

1 mole NaCN = 49.0073 grams NaCN

25.0 grams NaCN x (1 mole NaCN/49.0073 grams NaCN) = 0.510 moles NaCN

2. Determine the number of moles Au that will reacts with this moles NaCN.

Note: 4 moles Au reacts with 8 moles NaCN (from the balanced chemical equation)

0.510 moles NaCN x (4 moles Au/8 moles NaCN) = 0.255 moles Au

3. Convert moles Au to grams Au using its atomic mass.

Note: 1 mole Au = 196.9666 grams Au

0.255 moles Au x (196.9666 grams Au/1 mole Au) = 50.2 grams Au

b.

% Au = mass of Au/mass of sample x 100%

% Au = 50.2 grams/150.0 grams x 100% = 33%

75.

Steps:

1. Convert 0.275 grams AgBr to moles AgBr

Note: 1 mole AgBr = 187.7723 grams AgBr

0.275 grams AgBr x (1 mole AgBr/187.7723 grams AgBr) = 0.00146 mole AgBr

2. Determine the number of moles Na3Ag(S2O3)2 produced from this mole AgBr.

Note: 1 mole AgBr produces 1 mole Na3Ag(S2O3)2

0.00146 mole AgBr x (1 mole Na3Ag(S2O3)2/1 mole AgBr) = 0.00146 mole Na3Ag(S2O3)2

3. Convert to grams Na3Ag(S2O3)2

Note: 1 mole Na3Ag(S2O3)2 = 401.0961 grams Na3Ag(S2O3)2

0.00146 mole Na3Ag(S2O3)2 x (401.0961 grams Na3Ag(S2O3)2/1 mole Na3Ag(S2O3)2) = 0.586 grams Na3Ag(S2O3)2

That's it!n_n

• The easiest way to try this is to write down it out four three mol O2 4 1 mol clear up for that to get O2 in grams, than divide with the aid of the molar mass to get moles