chemistry calculation

Question

A math student need help with chemistry calculation. An Alloy consist of 4 parts of gold and 9 parts of silver. How much gold should be mixed with 360g of silver

Answer 1

Hi Carlene,

Let's look at the problem using the optimal set of steps that works well for most chem problems:

1. List the givens (this may sometimes be an inventory of species, such as Au and Ag).
2. Establish the goal, or final answer; note the units.
3. Set up an equation and path of solution steps.
4. Solve. Does the answer make sense?
5. Check. Plug in the numerical value, if applicable.

List: an alloy is 4 parts Au to 9 parts Ag.

What do parts mean? When we discuss "parts" in the context of "PPM," we refer to individual atoms or molecules, such as 1 part lead (Pb) per million parts of water = Pb is 1 PPM.

When we refer to parts, we use moles, such as 4 moles Au to 9 moles Ag.

Goal: find the mass of Au, which would be in g. Always look our for different units.

Equation: the given ratio is 4:9.

4 Au ⁄ 9 Ag

It's best to write the ratios vertically.

4 Au / 9 Ag = 4 mol Au / 9 mol Ag

Since the given Ag mass is 360g, what must we do with moles?

Convert the Au and Ag moles to grams. Use the periodic table for molar mass.

Now you have a ratio of the theoretical Au mass to Ag mass:

g Au / g Ag

Create a proportion (use =) with the next actual ratio of

x g Au / 360 g Ag

Do you remember how to solve for x?

Cross multiply (AKA cross products theorem from math).

Check the answer by plugging in.

*In chem, when you see a problem combining terms such as parts, percentage, mass, and moles, then expect to do conversions. Mass is typically given, such as the amount of a reactant. We must convert to moles, which is quantity of objects (e.g., molecules). Yep, this requires plenty of paper and writing to solve problems accurately.

Quantities are what we relate to each other in chemical reaction equations, such as 3 moles of hydrogen gas H₂ + 1 mole of nitrogen gas N₂ —> 2 moles ammonia NH₃.