Instruction

1

Task. Calculate the mass of aluminium sulfide if the reaction of sulfuric acid entered the 2.7 g of aluminium.

2

Recorded a short condition

This:

m(Al) =2, 7 g

H2SO4

Find:

m(Al2 (SO4) 3)-?

3

Before to solve problems in chemistry, a chemical equation. In the interaction of metals with dilute acid forming salt and released gaseous substance is hydrogen. We arrange the coefficients.

2Al + 3H2SO4 = Al2 (SO4) 3 + 3H2

When you always need to pay attention only to substances for which known and you need to find, options. All the rest are not taken into account. In this case it will be: Al and Al2 (SO4) 3

4

Find the relative molecular mass of these substances in the table of D. I. Mendeleev

Mr(Al) =27

Mr(Al2 (SO4) 3) =27•2(32•3+16•4•3) =342

Translate these values into molar mass (M) multiplied by 1 g/mol

M(Al) =27G/mol

M(Al2 (SO4) 3) =342g/mol

5

Recorded a basic formula, which connects the amount of substance (n), mass (m) and molar mass (M).

n=m/M

Carry out calculations according to the formula

n(Al) =2.7 g/27G/mol=0.1 mol

6

Make up two ratio. The first relation based on equation based on the odds faced by the formulas of the substances whose parameters are given or need to find.

First ratio: 2 mol Al you have 1 mole of Al2 (SO4) 3

The second ratio of 0.1 mol Al we have X mole Al2 (SO4) 3

(compiled on the basis of the obtained results)

Solve the proportion, given that X is the amount of substance

Al2 (SO4) 3 and has the unit mol

Here

n(Al2 (SO4) 3)=0.1 mol(Al)•1 mol(Al2 (SO4) 3):2моль Al=0.05 mol

7

Now there is the amount of substance and molar mass of Al2(SO4)3, consequently, you can find lots of which derive from the basic formula

m=nM

m(Al2 (SO4) 3)=0.05 mol•342g/mol=17,1 g

Recorded

Answer: m(Al2 (SO4) 3)=17,1 g

8

At first glance, it seems that to solve chemistry problems is very difficult, but it is not. And to verify the degree of assimilation, first try to solve the same problem, but only yourself. Then substitute other values using the same equation. And the last step is the solution of the new equation. And if you were able to handle what – you're to be congratulated!

Useful advice

Wonderful helper during solving tasks is a manual, time-tested "problems in chemistry for entering Universities" G. P. Khomchenko. And don't be afraid to use it – it proposes the solution of problems from the very beginning!

# Advice 2: How to solve problems with parameters

To solve the problem with the - a option means to find out what is the variable if any, or a specified parameter value. Or the task may be to search for those parameter values under which the variable meets certain conditions.

Instruction

1

If the given equation or inequality can be simplified, be sure to use it. Apply standard methods for solving equations, as if the argument were a normal number. As a result, you will be able to Express the variable via a parameter, for example, x=R/2. If the solution of equation you met with no restrictions on the parameter value (it is not necessary under the root sign, under the sign of the logarithm in the denominator), record the answer, putting that he found for all valid values of the parameter R.

2

For solving problems with standard charts (e.g., line, parabola, hyperbola) use the graphical method. Divide the area of parameter values for such intervals where the value of the variable (or variables) will be different, and for each interval, draw a line graph. Pay special attention to the extreme points of the lines to precisely determine their belonging to the schedule, substitute this value into the function and solve the equation with him. If the equation at this point has no solution (for example, is obtained by dividing by zero), eliminate it from the graph, noting an empty circle.

3

To solve the problem with respect to the parameter, pick the variable and the argument for equal members of equations or inequalities and simplify the expression. Then return to the original sense of the members and consider the solution

**of the problem**for all possible parameter values. For this set of parameter values you need to divide into intervals.4

When you search for the boundaries of intervals, pay attention to those expressions involving a parameter. For example, you have the expression (a-5), among the borders of intervals have to be number 5, since this value reverses the value in the parentheses to 0. Great importance is the expression under the sign of the parameter of division, root, module , etc.

5

When you find the limits of intervals, consider its function for each of them. To simplify this, just substitute into the function one of the numbers from this interval and solve the resulting problem. Often, just by substituting different values, it is possible to find the right way of solving the problem.